Though not definitive, the definition I've always used is -- Snapper is a blackjack (or "21") game term which describes an initial 2-card blackjack hand -- an ace plus a 10-value card (a 10 or face card). Such a hand receives a 3-to-2 bet payment at full-pay blackjack tables. So a $5 bet would be paid off by the dealer with a $5 chip and a $2.50 chip, assuming they use $2.50's.

So the $2.50 chip is associated with a natural blackjack.

The next question is "why the snapper term"? Here, there's no definitive answer, though some believe it comes from the sound the cards make when a player peeks at them and discovers, excitedly, that they scored a blackjack. Or it's the sound made when the player slaps the 2 cards down on the table, showing all that it's a blackjack. This applies to the face-down version of blackjack.

The term had been around quite a while -- probably 20+ years, and maybe much longer.

You won't hear the snapping sound as much these days. Many tables deal the cards face-up now, and besides, the house discourages anything that would bend the cards. For an example or the snapping sound, check out the baccarat tables sometime and watch the players squeeze their dealt cards. A natural 8 or 9 will produce such a sound at times.