A horny, super quick sexual orgasm.
Can be self-induced (masturbation), acheived with another (sex - anal/oral/handjob/blowjob/etc), or with a group (orgy).
Usually said in informal terms. Most commonly referred to when talking about party sex or in a joke. (ie: It never actually happened and is used in a sexual joke, or it was very meaningless at an orgy.)
Girl 1 : "Oh my gosh, I had the BEST sponza last night!"
Girl 2: "I bet it was horny and explosive!"
Girl 3: "Oh, it was."
Girl 2: "Oohhhh, I see how it is."
Boy 1: "Ohh yeah, I just had an amazing sponza. Better than my girlfriend!"
Boy 2: "Did you just do that now in the bathroom? That sure was fast."
Boy 1: "That's all it takes for a sponza."