Wiktionary
n. A genre of pornography in which a person consumes the semen of multiple men, usually from some vessel or container such as a cup or beaker.
Wikipedia
is a Japanese term for sexual activity in which a person consumes the semen of one or more men, usually from some kind of container. Commonly used containers in this genre include cups, large beakers, bowls, baking pans, and wine or cocktail glasses. The vast majority of these scenarios involve the semen of multiple men. Pornographic films with gokkun scenes have had up to 200 men participating.
"Gokkun" can also refer to the sexual act of swallowing semen after performing fellatio or participating in a bukkake. The word "gokkun" is onomatopoetic, and translates roughly as the English word "gulp", the sound made by swallowing.