This glossary defines specialized terminology you may encounter while using Xapian. Some of the entries are standard in the field of Information Retrieval, while others have a specific meaning in the context of Xapian.

The weighting scheme which Xapian uses by default. BM25 is a refinement on the original probabilistic weighting scheme, and recent TREC tests have shown BM25 to be the best of the known probabilistic weighting schemes. It's sometimes known as "Okapi BM25" since it was first implemented in an academic IR system called Okapi.
Boolean Retrieval
Retrieving the set of documents that match a boolean query (e.g. a list of terms joined with a combination of operators such as AND, OR, AND_NOT). In many systems, these documents are not ranked according to their relevance. In Xapian, a pure Boolean query may be used, or alternatively a Boolean style query can filter the retrieved documents, which are then ranked using a weighting formula.
Brass was the current "under development" database format in Xapian 1.2.x, 1.3.0 and 1.3.1. It was renamed to 'glass' in Xapian 1.3.2 because we decided to use backend names in ascending alphabetical order to make it easier to understand which backend is newest, and since 'flint' was used recently, we skipped over 'd', 'e' and 'f'.
Chert was the stable database format used in Xapian 1.2.x. It is similar to Flint in many ways, but generally faster, and uses significantly less disk space. Chert is very efficient and highly scalable. It supports incremental modifications, and concurrent single-writer and multiple-reader access to a database.
Collection Frequency
The collection frequency of a term is the total number of times is occurs in the database. This is equal to the sum of the within-document frequency for the term in all the documents it occurs in.
In Xapian (as opposed to a relational database system) a database consists of little more than indexed documents: this reflects the purpose of Xapian as an information retrieval system, rather than an information storage system. These may also occasionally be called Indexes. Glass is the default backend used by Xapian 1.4; Chert was the default backend used by Xapian 1.2; Flint was used by Xapian 1.0; Quartz was used prior to Xapian 1.0.
Document ID
A unique positive integer identifying a document in a Xapian database.
Document data
The document data is one of several types of information that can be associated with each document, the contents can be set to be anything in any format, examples include fields such as URL, document title, and an excerpt of text from the document. If you wish to interoperate with Omega, it should contain name=value pairs, one per line (recent versions of Omega also support one field value per line, and can assign names to line numbers in the query template).
These are the items that are being retrieved. Often they will be text documents (e.g. web pages, email messages, word processor documents) but they could be sections within such a document, or photos, video, music, user profiles, or anything else you want to index.
Edit distance
A measure of how many "edits" are required to turn one text string into another, used to suggest spelling corrections. The algorithm Xapian uses counts an edit as any of inserting a character, deleting a character, changing a character, or transposing two adjacent characters.
ESet (Expand Set)
The Expand Set (ESet) is a ranked list of terms that could be used to expand the original query. These terms are those which are statistically good differentiators between relevant and non-relevant documents.
Flint was the default database format used in Xapian 1.0.x. It was deprecated in 1.2.x and removed in 1.3.0.
Glass is the default database format in Xapian 1.4.
If a document is described by a term, this term is said to index the document. Also, the database in Xapian and other IR systems is sometimes called an index (by analogy with the index in the back of a book).
The indexer takes documents (in various formats) and processes them so that they can be searched efficiently, they are then stored in the database.
Information Need
The information need is what the user is looking for. They will usually attempt to express this as a query string.
Information Retrieval (IR)
Information Retrieval is the "science of search". It's the name used to refer to the study of search and related topics in academia.
MSet (Match Set)
The Match Set (MSet) is a ranked list of documents resulting from a query. The list is ranked according to document weighting, so the top document has the highest probability of relevance, the second document the second highest, and so on. The number of documents in the MSet can be controlled, so it does not usually contain all of the matching documents.
Normalised document length (ndl)
The normalised document length (ndl) is the length of a document (the number of terms it contains) divided by the average length of the documents within the system. So an average length document would have ndl equal to 1, while shorter documents have ndl less than 1, and longer documents greater than 1.
Omega comprises two indexers and a CGI search application built using the Xapian library.
Posting List
A posting list is a list of the documents which a specific term indexes. This can be thought of as a list of numbers - the document IDs.
An instance of a particular term indexing a particular document.
Precision is the density of relevant documents amongst those retrieved: the number of relevant documents returned divided by the total number of documents returned.
Probabilistic IR
Probabilistic IR is retrieval using a weighting formula derived from probability theory to produce a ranked list of documents based upon estimated relevance. Xapian supports several families of weighting schemes, some of which are based on probabilistic methods.
Quartz was the database format used by Xapian prior to version 1.0. Support was dropped completely as of Xapian 1.1.0.
A query is the information need expressed in a form that an IR system can read. It is usually a text string containing terms, and may include Boolean operators such as AND or OR, etc.
Query Expansion
Modifying a query in an attempt to broaden the search results.
RSet (Relevance Set)
The Relevance Set (RSet) is the set of documents which have been marked by the user as relevant. They can be used to suggest terms that the user may want to add to the query (these terms form an ESet), and also to adjust term weights to reorder query results.
Recall is the proportion of relevant documents retrieved - the number of relevant documents retrieved divided by the total number of relevant documents.
Essentially, a document is relevant if it is what the user wanted. Ideally, the retrieved documents will all be relevant, and the non-retrieved ones all non-relevant.
The searcher is a part of the IR system, it takes queries and reads the database to return a list of relevant documents.
A stemming algorithm performs linguistic normalisation by reducing variant forms of a word to a common form. In English, this mainly involves removing suffixes - such as converting any of the words "talking", "talks", or "talked" to the stem form "talk".
Stop word
A word which is ignored during indexing and/or searching, usually because it is very common or doesn't convey meaning. For example, "the", "a", "to".
Xapian can store synonyms for terms, and use these to implement one approach to query expansion.
Term List
A term list is the list of terms that index a specific document. In some systems this may be a list of numbers (with each term represented by a number internally), in Xapian it is a list of strings (the terms).
Term frequency
The term frequency of a specific term is the number of documents in the system that are indexed by that term.
A term is a string of bytes (often a word or word stem) which describes a document. Terms are similar to the index entries found in the back of a book and each document may be described by many terms. A query is composed from a list of terms (perhaps linked by Boolean operators).
Term Prefix
By convention, terms in Xapian can be prefixed to indicate a field in the document which they come from, or some other form of type information. The term prefix is usually a single capital letter.
Test Collection
A test collection consists of a set of documents and a set of queries each of which has a complete set of relevance assignments - this is used to test how well different IR methods perform.
A standard variable-length byte-oriented encoding for Unicode.
A discrete meta-data attribute attached to a document. Each document can have many values, each stored in a different numbered slot. Values are designed to be fast to access during the matching process, and can be used for sorting, collapsing redundant documents, implementing ranges, and other uses. If you're just wanting to store "fields" for displaying results, it's better to store them in the document data.
Within-document frequency (wdf)
The within-document frequency (wdf) of a term in a specific document is the number of times it is pulled out of the document in the indexing process. Usually this is the size of the wdp vector, but in Xapian it can exceed it, since we can apply extra wdf to some parts of the document text.
Within-document positions (wdp)
In the case where a term derives from words actually in the document, the within-document positions (wdp) are the positions at which that word occurs within the document. So if the term derives from a word that occurs three times in the document as the fifth, 22nd and 131st word, the wdps will be 5, 22 and 131.
Within-query frequency (wqf)
The within-query frequency (wqf) is the number of times a term occurs in the query. This statistic is used in the BM25 weighing scheme.