1. In geology, a blowhole may be a hole in a cliff or cave through which seawater spurts at high tide. Blowholes are formed by erosion. Continued hydraulic action by pounding waves on the roof of the cave forms a sort of chimney. Eventually this breaks through the surface of the ground near the edge of the cliff. At high tide, incoming waves force water out of the top of the blowhole.
2. In terms of animal anatomy, a blowhole is the nostrils (single or double) of whales, situated toward the back of the skull.