noun.
As much as a teaspoon holds; as a definite quantity, a fluidrachm. When solids are measured by the teaspoonful, the spoon is generally heaped.
the GNU version of the Collaborative International Dictionary of English
noun.
As much as teaspoon will hold; enough to fill a teaspoon. In cooking, it is usually estimated as 4.9 milliliters, which is 1/6 of a fluid ounce, or 1/3 of a tablespoonful. In cooking recipes it may be abbreviated as t.