Convex & Concave Hairstyles
The "convex haircut" is a layered-cut that creates outward movement in the hairstyle. It can be cut to have clear, hard lines, or very subtle soft lines, but will always result in an outward movement of the hair. An example of a convex hairstyle would be the classic Farrah Fawcett "tousled" look and the many modern variations.
The "concave" haircut has the opposite movement. The hair is cut with hard or soft lines to curve inward. Examples of this are the classic chin-length bob where the hair tapers inward, and styles like the "Dorothy Hamill" and the "Purdy" where the hair is sharply stacked in tapering layers, or over-directed to a stationary weight line.
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See also:
Can you explain the terms square, round, concave, and convex layers?
How to cut a dramatic A-line or concave bob
Dorothy Hamill hairstyles