Hemistich
Each of the two halves of a line cut by the caesura — six syllables in the classical alexandrine.
The hemistich (Greek for "half-line") names each of the two parts of a verse separated by the caesura. In the classical alexandrine, two equal six-syllable hemistichs; in the traditional decasyllable, most often a 4 // 6 split.
The hemistich is the long line's breathing unit: the voice leans on it, meaning organizes around it. The swing of two symmetrical hemistichs naturally carries antithesis — the line's grammar then weds rhetoric. Breaking that expected symmetry, by straddling the caesura or deliberate imbalance, is one of modern verse's major expressive gestures.
Example
In Old English verse, each line splits into two half-lines bound by alliteration.