Definition
Stemming
Stemming reduces word forms to shared matching forms, called stems, using language-dependent rules such as removing prefixes or suffixes. A stem need not be a dictionary word or a true linguistic root.
Comparison with Lemmatisation
Lemmatisation seeks the appropriate dictionary headword. Stemming instead uses approximate transformations of the written form, without requiring the result to be a dictionary entry.
Use in Retrieval
Stems can serve as indexed terms. Apply the same stemming rules to indexed text and query words so intended variants match.
Example
A stem rather than a headword
A stemmer
may produce The shared output automat is a matching form, not the dictionary headword of each input.
A shared stem
Here, the shared stem also happens to be a dictionary word.