A person's hairline when it starts to recede and he (or she) loses two clumps of hair on the top of his (or her) head and at the sides, been left with a piece of hair in the middle of the head which (usually when brushed or slicked back) resembles a triangle.

ie: Phil Collins from the late 80's onwards.
Do you remember that guy from the film, you know, he had a three degree's triangle?
by Anthony Corbett December 1, 2006
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