The Collaborative International Dictionary of English v.0.48:

Stemmer \Stem"mer\, n.
   One who, or that which, stems (in any of the senses of the
   verbs).
   [1913 Webster]

WordNet (r) 3.0 (2006):

stemmer
    n 1: a worker who strips the stems from moistened tobacco leaves
         and binds the leaves together into books [syn: stripper,
         stemmer, sprigger]
    2: a worker who makes or applies stems for artificial flowers
    3: an algorithm for removing inflectional and derivational
       endings in order to reduce word forms to a common stem [syn:
       stemmer, stemming algorithm]
    4: a miner's tamping bar for ramming packing in over a blasting
       charge
    5: a device for removing stems from fruit (as from grapes or
       apples)

The Free On-line Dictionary of Computing (30 December 2018):

stemmer
stemming

    A program or algorithm
   which determines the morphological root of a given inflected
   (or, sometimes, derived) word form -- generally a written word
   form.

   A stemmer for English, for example, should identify the
   string "cats" (and possibly "catlike", "catty" etc.) as
   based on the root "cat", and "stemmer", "stemming", "stemmed"
   as based on "stem".

   English stemmers are fairly trivial (with only occasional
   problems, such as "dries" being the third-person singular
   present form of the verb "dry", "axes" being the plural of
   "ax" as well as "axis"); but stemmers become harder to design
   as the morphology, orthography, and character encoding of
   the target language becomes more complex.  For example, an
   Italian stemmer is more complex than an English one (because
   of more possible verb inflections), a Russian one is more
   complex (more possible noun declensions), a Hebrew one is even
   more complex (a hairy writing system), and so on.

   Stemmers are common elements in query systems, since a user
   who runs a query on "daffodils" probably cares about documents
   that contain the word "daffodil" (without the s).

   (This dictionary has a rudimentary stemmer which currently
   (April 1997) handles only conversion of plurals to singulars).

   (1997-04-09)