Definition of "caesural"

By the third decade of the eighteenth century, the syllabic line that really threatened to stay was an uncouth thing of thirteen syllables (counting the obligative feminine terminal), with a caesura after the seventh syllable: […] The order of the stresses in the thirteener went in jumps and jolts and varied from line to line. The only rule (followed only by purists) was that the seventh, caesural, syllable must bear a beat.

1964, Vladimir Nabokov, “The Origination of Metrical Verse in Russia”, in Notes on Prosody […] (Bollingen Series; LXXIIa), New York, N.Y.: Bollingen Foundation, page 35