Source : Webster's Revised Unabridged Dictionary (1913)

Teaspoonful \Tea"spoon`ful\, n.; pl. {Teaspoonfuls}.
   As much as teaspoon will hold; enough to fill a teaspoon; --
   usually reckoned at a fluid dram or one quarter of a
   tablespoonful.

Source : WordNet®

teaspoonful
     n : as much as a teaspoon will hold [syn: {teaspoon}]