A teaspoon is a unit of volume measure equal to 1/3 tablespoon. It is either approximately or (in some countries) exactly equal to 5 mL. In the USA there are 16 teaspoons in 1/3 cup, and there are 6 teaspoons in 1 fluid ounce.
"Teaspoon" may be abbreviated as t (note: lowercase letter t) or tsp.
A small spoon, as might be used for eating yogurt from a small container or adding sugar to tea, is about 1 teaspoon in size.