n.; pl. Teaspoonfuls r Teaspoonsful. 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. In the 1890 dictionary, it was defined as a fluid dram or one quarter of a tablespoonful. [ 1913 Webster +PJC ]