(n.) A small glass designed to hold a single measure of liquor, usually 1.5 oz. It can be used either as a drinking vessel or as a measuring tool. Shot glasses are typically made of thick glass with a strong base to prevent shattering, should a drinker feel the need to slam their shot on the bar after a drink.
The common misconception is that a shot is actually only 1 oz, as many standards for what constitutes a legal "drink" deem a shot of that size "one drink." But since your standard shot glass is a bit more than that, it's important to keep that factor in mind when assessing how much time would have to pass in order for the alcohol to filter out of your system. Two shots, in 1.5 oz shot glasses, would actually constitute three legal drinks and take at least three hours to filter out of your system.
But don't let the misconception stop you from having a fun night of shots. Some of the best spirit and alcohol combinations are meant to be pounded back. Just know that shot glasses vary in shape, size and pour. Use this newfound information to plan your night accordingly and drink responsibly. And maybe give yourself an hour or two more than you think you need.