Gibbous describes the phase of the moon when more than half but not all of it appears lit up in the sky. If you look up on a clear night and see the moon looking almost full but with a small dark sliver still visible on one side, you're seeing a gibbous moon.

The moon goes through phases as it orbits Earth, and gibbous is the phase between half-moon and full moon. When the lit portion is growing larger each night, astronomers call it waxing gibbous. After the full moon, when the lit part shrinks back down, it's called waning gibbous. The word comes from Latin meaning “humpbacked,” which makes sense when you notice how the moon seems to bulge outward during this phase.

You might encounter this word in a spooky story set on a night with a gibbous moon, when the moonlight is bright enough to cast shadows but the darkness isn't completely banished. The gibbous phase lasts about a week, so if you start watching the moon regularly, you'll see it move through this shape twice each month.