[[stemming]]
=== Stemming

_Stemming_ is the process of reducing a word to its root form. This ensures
variants of a word match during a search.

For example, `walking` and `walked` can be stemmed to the same root word:
`walk`. Once stemmed, an occurrence of either word would match the other in a
search.

Stemming is language-dependent but often involves removing prefixes and
suffixes from words.

In some cases, the root form of a stemmed word may not be a real word. For
example, `jumping` and `jumpiness` can both be stemmed to `jumpi`. While `jumpi`
isn't a real English word, it doesn't matter for search; if all variants of a
word are reduced to the same root form, they will match correctly.

[[stemmer-token-filters]]
==== Stemmer token filters

In {es}, stemming is handled by stemmer <<analyzer-anatomy-token-filters,token
filters>>. These token filters can be categorized based on how they stem words:

* <<algorithmic-stemmers,Algorithmic stemmers>>, which stem words based on a set
of rules
* <<dictionary-stemmers,Dictionary stemmers>>, which stem words by looking them
up in a dictionary

Because stemming changes tokens, we recommend using the same stemmer token
filters during <<analysis-index-search-time,index and search analysis>>.

[[algorithmic-stemmers]]
==== Algorithmic stemmers

Algorithmic stemmers apply a series of rules to each word to reduce it to its
root form. For example, an algorithmic stemmer for English may remove the `-s`
and `-es` prefixes from the end of plural words.

Algorithmic stemmers have a few advantages:

* They require little setup and usually work well out of the box.
* They use little memory.
* They are typically faster than <<dictionary-stemmers,dictionary stemmers>>.

However, most algorithmic stemmers only alter the existing text of a word. This
means they may not work well with irregular words that don't contain their root
form, such as:

* `be`, `are`, and `am`
* `mouse` and `mice`
* `foot` and `feet`

The following token filters use algorithmic stemming:

* <<analysis-stemmer-tokenfilter,`stemmer`>>, which provides algorithmic
stemming for several languages, some with additional variants.
* <<analysis-kstem-tokenfilter,`kstem`>>, a stemmer for English that combines
algorithmic stemming with a built-in dictionary.
* <<analysis-porterstem-tokenfilter,`porter_stem`>>, our recommended algorithmic
stemmer for English.
* <<analysis-snowball-tokenfilter,`snowball`>>, which uses
https://snowballstem.org/[Snowball]-based stemming rules for several
languages.

[[dictionary-stemmers]]
==== Dictionary stemmers

Dictionary stemmers look up words in a provided dictionary, replacing unstemmed
word variants with stemmed words from the dictionary.

In theory, dictionary stemmers are well suited for:

* Stemming irregular words
* Discerning between words that are spelled similarly but not related
conceptually, such as:
** `organ` and `organization`
** `broker` and `broken`

In practice, algorithmic stemmers typically outperform dictionary stemmers. This
is because dictionary stemmers have the following disadvantages:

* *Dictionary quality* +
A dictionary stemmer is only as good as its dictionary. To work well, these
dictionaries must include a significant number of words, be updated regularly,
and change with language trends. Often, by the time a dictionary has been made
available, it's incomplete and some of its entries are already outdated.

* *Size and performance* +
Dictionary stemmers must load all words, prefixes, and suffixes from its
dictionary into memory. This can use a significant amount of RAM. Low-quality
dictionaries may also be less efficient with prefix and suffix removal, which
can slow the stemming process significantly.

You can use the <<analysis-hunspell-tokenfilter,`hunspell`>> token filter to
perform dictionary stemming.

[TIP]
====
If available, we recommend trying an algorithmic stemmer for your language
before using the <<analysis-hunspell-tokenfilter,`hunspell`>> token filter.
====

[[control-stemming]]
==== Control stemming

Sometimes stemming can produce shared root words that are spelled similarly but
not related conceptually. For example, a stemmer may reduce both `skies` and
`skiing` to the same root word: `ski`.

To prevent this and better control stemming, you can use the following token
filters:

* <<analysis-stemmer-override-tokenfilter,`stemmer_override`>>, which lets you
define rules for stemming specific tokens.
* <<analysis-keyword-marker-tokenfilter,`keyword_marker`>>, which marks
specified tokens as keywords. Keyword tokens are not stemmed by subsequent
stemmer token filters.
* <<analysis-condition-tokenfilter,`conditional`>>, which can be used to mark
tokens as keywords, similar to the `keyword_marker` filter.


For built-in <<analysis-lang-analyzer,language analyzers>>, you also can use the
<<_excluding_words_from_stemming,`stem_exclusion`>> parameter to specify a list
of words that won't be stemmed.
