Noun, almost always plural. The white, foamy tops of waves out on the water — the frothy crest you see when the wind is strong enough to break the top of a wave over. So whitecaps are a sign to read: calm water has none; when whitecaps appear, the wind has picked up, and sailors, swimmers and small-boat fishermen take that as a warning.

The usual patterns are "there were whitecaps on the lake" and "the sea was covered in whitecaps". You can also use it as an adjective-ish description: "whitecapped waves". The singular whitecap exists but sounds odd on its own — the whole point is that there are many of them scattered across the surface.

The dictionary sends you to 'white horses' because that is the British word for exactly the same thing — British speakers see galloping white horses in the foam, American speakers see white caps (hats). Whitecaps is the American and international word and is by far the more common of the two now; white horses sounds a little old-fashioned and poetic. Both are everyday words, not technical ones — a weather forecaster or a sailing instructor will say them without explanation.