Grammar. Of an accent, a syllable: Unstressed.

Of a syllable (esp. in poetry): unstressed; pronounced without emphasis. Hence of a rhythm: consisting mainly of such syllables.

Linguistics and Prosody. Of a word, syllable, etc.: pronounced without stress, unstressed.

Prosody. Not having an accent, unaccented; usually, not bearing the stress or syllabic accent, as ‘the atonic syllables of a word,’ ‘an atonic vowel’.

(un-, prefix1 affix 2.)