A V-shaped dip in the hairline in the middle of the forehead, as opposed to a hairline that runs across the forehead in a straight line. Vegeta, Mishima Kazuya, and the Joker are prominent examples of characters with distinctive widow's peaks.
A widow's peak is a natural facial feature. Don't use this tag just because of the way the hair is styled, e.g., because the character has hair intakes or a lock of hair lying across their forehead. This tag is not exclusive with bangs, but enough of the hairline needs to be visible to confirm that a widow's peak is in fact present.