When your girlfriend (or boyfriend if you swing that way) stops mid way through giving you a blowjob to kiss you on the lips.

A full moon kiss would be when they kiss you on the lips, after they have given the blowjob.

The former is arguably preferable.
Guy 1: "Dude, Alice is so good at giving head. She even stopped half way through and gave me a half moon kiss!"

Guy 2: "Nice!"
by MichaelToTheJ September 26, 2012
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