A teaspoon (tsp) is the smallest common cooking measure in the US customary system: 1/3 of a tablespoon, 1/6 of a fluid ounce and 1/48 of a US cup. Like the tablespoon it is a fixed volume, unlike the teaspoons in a cutlery drawer.
Conversion
3 tsp = 1 tbsp
6 tsp = 1 fl oz
48 tsp = 1 cup
mL = tsp × 4.928922
NIST lists 4.928922 mL per teaspoon. The FDA's nutrition-label rule (21 CFR 101.9(b)(5)(viii)) uses 5 mL.
Example
A recipe that asks for 4 teaspoons of spice needs 4 ÷ 3 = 1.33 tablespoons, which is 1 tablespoon plus 1 teaspoon. In milliliters, 4 × 4.928922 = 19.7 mL.
The teaspoons in a tablespoon guide has a full chart, and the converters tsp to tbsp and tsp to ml handle other amounts.
Common mistakes
A "teaspoon" in a recipe is the measuring spoon, not the flatware teaspoon. Level the spoon with a knife edge for dry ingredients, and keep tsp and tbsp apart: confusing them is a three-fold error, which matters most for salt, baking soda and strong spices.