Unit Definition (Teaspoon [US])
The teaspoon is a unit of volume used in food recipes. The U.S. teaspoon is equal to 1/3 tablespoon or 1/48 cup; this is equivalent to 1/6 fluid ounce, about 0.30 cubic inches, or approximately 4.9 milliliters.

Unit Definition (Teaspoon [UK])
The teaspoon is a unit of volume used in food recipes. In Britain, a traditional teaspoonful in the kitchen was equal to 1/8 Imperial fluid ounce or approximately 3.55 milliliters, but the medical teaspoonful was usually 5 milliliters.