The Adventurers looked down over the rocky decline that separated them from the stronghold of the Cabal. A wisp of pale smoke drifted up from a round ventilation hole, marking the spot.

“I think I see a path,” Francis Hebert said. “See there?”

Max shielded his eyes. “Dammit, I can’t tell whether that’s concave or convex. This place is crazier than chopsticks for a snake.”

Larry Niven And Steve Barnes, The Barsoom Project, 1989.

Concave means ‘having an outline or surface curved like the interior of a circle or sphere’. Convex means ‘having an outline or surface curved like the exterior of a circle or sphere’. [The New Fowler’s Modern English Usage, Revised 3rd ed.]
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