Teaspoon

In some countries, a teaspoon full is a unit of volume, especially in cooking recipes and pharmaceutic prescriptions.  In English it is abbreviated as t., ts., tsp. or tspn. ), never capitalized, as capitals are customarily reserved for the larger tablespoon ("T." or "Tbls." or "Tb.").  In German and Dutch teaspoon is abbreviated TL, for Teelöffel and Theelepel respectively.

In the United States one teaspoon is 1⁄3 tablespoon, 1⁄6 U.S. fl. oz, 1⁄48 of a cup, and 1⁄768 of a U.S. liquid gallon. This is approximately 5 mL and 1⁄3 of a cubic inch. For nutritional labeling purposes on food packages in the U.S., the teaspoon is, by federal regulations, rounded to precisely 5 mL.

Common teaspoons such as bar spoons for measuring ingredients and stirring mixed drinks are often not designed to contain a standard volume.  In practice, they may hold anything between 2.5 mL and 6 mL of liquid, so caution must be employed when using a teaspoon to measure a prescribed dose of medicine. For this reason and in order to avoid dispensing errors, special measuring spoons are available that hold exactly 5 mL.

If a recipe calls for a level teaspoonful of a dry ingredient (salt, flour, etc.), this refers to an approximately leveled filling of the spoon, producing the same volume as for liquids.
A rounded teaspoonful is a larger but less precise measure, produced without leveling the ingredient off nor heaping it as high as possible.

A heaping (American English) or heaped (British English) teaspoonful is a larger inexact measure, equal to the most that can be obtained by scooping the dry ingredient up without leveling it off. For some ingredients, e.g. flour, this quantity can vary considerably.
When no particular type of teaspoonful is specified for a dry ingredient it may mean a level or a rounded spoonful, never a heaping/heaped one.