A (grammatical) marker (a cover term) is a °clitic or an °affix.
Alternatively and equivalently, one could say that "a grammatical marker is a bound morph that is not a root".
Linguists almost never discuss the meaning of "(grammatical) marker", but the term is very widely used, especially in the description of non-European languages, where it is taken as a cover term for an affix and a function word (= clitic). Matthews's (2007) definition (as the "realization of a unit") is too vague, and also too broad because one would say, for example, that plural can be "realized" as vowel change, but vowel change would not be considered as a marker.
A synonymous term may be "grammatical form" (though is is rarely used in a technical sense), and the term "gram" from Bybee et al. (1994) also has a similar meaning.
"marker: Any unit or feature conceived (a) as the realization of a unit etc. at a more abstract level of representation, or (b) as indicating something to a potential addressee. Thus a ‘case marker’ or ‘tense marker’ is an inflection etc. which realizes a case or a tense; a boundary marker indicates a boundary e.g. between words; the in the house ‘marks’ the phrase as definite. A social marker likewise gives an indication of the status of a speaker in society." (Matthews 2007)
"Lexical forms include items such as nouns, verbs, adjectives, and adverbs. Grammatical forms, or “grams” (Bybee et al. 1994), in contrast, roughly correspond to what is referred to in some other traditions as “functional categories”, like adpositions, articles, markers of tense, aspect and modality, and clitics or affixes used for word derivation or inflection." (Heine et al. 2026: