Everything:
iamb [ˈaɪæm ˈaɪæmb], iambus [aɪˈæmbəs]
n pl iambs, iambi [aɪˈæmbaɪ], iambuses Prosody
1. (Literature / Poetry) a metrical foot consisting of two syllables, a short one followed by a long one (˘ ¯)
2. (Literature / Poetry) a line of verse of such feet
[C19 iamb, from C16 iambus, from Latin, from Greek iambos]