Curve that's high and outside?

ARCH

Think of a structural shape that bends upward like a rainbow or a bridge’s support.

This clue plays on two meanings of "curve." In baseball, a pitch that's "high and outside" is a curveball thrown off the plate, while "arch" is another word for a curved shape. The pun works because "arch" sounds like it could describe that kind of pitch, even though it’s actually a structural or geometric term. It’s a clever way to misdirect with sports lingo before landing on the simpler definition.