An entry that begins before sam, so the phrase is already in motion when the cycle turns. Anagat ("not yet come") names the anticipation: the phrase starts while sam is still ahead, and the count is arranged so the true landing arrives exactly on the sam it straddles.
Anagat placement is deliberate suspense: the listener feels the phrase lean across the bar line. It demands exact arithmetic, since starting early by even half a matra without adjusting the tail throws the resolution off audibly.