The "Crop Top"

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Or at least this is what they used to call it.  The hair on top of the head is cut "close cropped" and one look makes this phrase self-explanatory.  For a modern twist, as well as a retro one, Matt Damon has chosen to wear this along with a regular haircut on the sides and back, making this style far from "military".   It's all 'good' for his 'identity'.