In linguistics, a marker is a free or bound morpheme that indicates the grammatical function of the marked word, phrase, or sentence.
For example, in Latin, a highly fusional language, the word amō ("I love") is marked by suffix -ō for indicative mood, active voice, first person, singular, present tense.
An unmarked form is the basic "neutral" form of a word, typically used as its dictionary lemma, such as—in English—for nouns the singular (e.g. cat versus cats), and for verbs the infinitive (e.g. to eat versus eats, ate and eaten).
Conversely, a marked form may happen to have a zero affix, like the genitive plural of some nouns in Russian (e.g. сапо́г).
In some languages, the same forms of a marker have multiple functions, such as when used in different cases or declensions (for example -īs in Latin).