noun
head sea; oncoming waves
Nautical term for waves coming from ahead, opposing a vessel's course.
向かい波の中を船が進むのは大変だ。
It's tough for a ship to make headway in a head sea.
追い波 refers to following seas (waves from behind), while 向かい波 refers to head seas (waves from ahead).
Compound of 向かい (mukai, 'facing, opposite') + 波 (nami, 'wave').