1) a. marked by convexity or swelling 

b. of the moon or a planet : seen with more than half but not all of the apparent disk illuminated

2) having a hump : humpbacked

In Context

About one-quarter up from the horizon to the point overhead will be a waxing gibbous moon, illuminated 93 percent by the sun.”

Joe Rao,, “The Moon Has a Close Encounter with Saturn on Tuesday Night,” 23 July 2018