A tablespoonful is the amount of something that fills a tablespoon, which is a large spoon used for serving food and measuring ingredients in cooking. When a recipe calls for “a tablespoonful of sugar,” it means enough sugar to fill that spoon level with its rim (not heaped up into a little mountain).

In cooking, a tablespoonful is a standard measurement equal to three teaspoonfuls. If you're making cookies and need to add vanilla extract, you might measure out a tablespoonful. Bakers and cooks use tablespoons so often that recipes usually abbreviate it as “tbsp” or just “T.”

The word can describe any substance measured this way: a tablespoonful of honey, a tablespoonful of medicine, or a tablespoonful of peanut butter.

When you see tablespoonful in older recipes or medicine instructions, it means exactly what it says: fill the spoon level to measure, then add that amount to whatever you're making.