| Definition -
Any morpheme whose primary role is grammatical, i.e. whose main role is to show how words are related to other words.
As opposed to morphemes whose primary role is semantic, i.e. whose main role is to deliver meaning.
For example, in English the grammatical morphemes are either:
(1) grammatical words (e.g. the articles the and a or prepositions such as to and in) or
(2) grammatical inflections (e.g. the past tense -ed, as in liked, or the possessive s, as in Albert's).