An eighth of a teaspoon is usually regarded in culinary measurements as .5 milliliters and is called a pinch. The United States customary unit for a teaspoon is equivalent to a third of a tablespoon, 4.928921599375 milliliters, 1/6 of a U.S. fluid ounce or 1/48 of a U.S. cup.
The term teaspoonful is often used as a unit of measurement in some countries. As a unit of measurement, the teaspoonful is mostly used to describe a particular amount used in cooking or for pharmaceutical preparations. It uses the abbreviation tsp or sometimes just t or tspn, always in lowercase letters, as the capital "T" is reserved for the tablespoon, the bigger unit of measurement also abbreviated as Tbsp.
Nutritional labeling laws in the United States define the teaspoon as being exactly 5 milliliters. It is also worth noting that the measurements using typical cutlery teaspoons can vary greatly, from 2.6 ml to 6 ml. When precision is important, such as when measuring medicine, it is a good idea to use measuring spoons instead of cutlery teaspoons.
There are also other definitions for teaspoon and these are level, rounded and heaping. A level teaspoon measurement is used for dry ingredients such as salt or pepper, and this means that the teaspoon is filled to level. A rounded teaspoon is about a scoop of a dry ingredient without leveling the spoon, and a heaping teaspoon means the maximum amount of dry ingredient that can be scooped without leveling.