of the word itself. Stemming algorithms, also known as stemmers, typically use a collection of simple rules to remove suffixes intended to model the language’s inflection rules. More advanced methods, lemmatisation methods, group together the inflected forms of a word through more complex rule sets based on a word’s part of speech or its record in a lexical database, transforming an inflected word through lookup or a series of transformations to its lemma. For a long time, it was taken to be proven that morphological normalisation by and large did not help retrieval performance. Once the attention of the information retrieval field moved to languages other than English, it was found that for some languages there were obvious gains to be found. Entity recognition Entity recognition is the process of locating and classifying entities within a text string. Named-entity recognition specifically focuses on named entities, such as names of people, places, and organizations. In addition, entity recognition includes identifying concepts in queries that may be represented by multi-word phrases. Entity recognition systems typically use grammar-based linguistic techniques or statistical machine learning models. Query rewriting Query rewriting is the process of automatically reformulating a search query to more accurately capture its intent. Query expansion adds additional query terms, such as synonyms, in order to retrieve more documents and thereby increase recall. Query relaxation removes query terms to reduce the requirements for a document to match the query, thereby also increasing recall. Other forms of query rewriting, such as automatically converting consecutive query terms into phrases and restricting query terms to specific fields, aim to increase precision. Apache Lucene search engine uses query rewrite to transform complex queries to more primitive queries, such as expressions with wildcards (e.g. quer*) into a boolean query of the matching terms from the index (such as query OR queries). See also Daniel Tunkelang's blog on Query Understanding ACM SIGIR 2010 Workshop Report on Query Representation and Understanding Proceedings of ACM SIGIR 2011 Workshop on Query Representation and Understanding ACM WSDM 2016 Workshop on Query Understanding for Search on All Devices Query Understanding for Search Engines (Yi Chang and Hongbo Deng, Eds.) References Information retrieval techniques Natural language processing