The Collaborative International Dictionary of English v.0.48:

Teaspoonful \Tea"spoon`ful\ (t[=e]"sp[=oo]n`f[.u]l), n.; pl.
   Teaspoonfuls (t[=e]"sp[=oo]n`f[.u]lz) or 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.

   Note: In the 1890 dictionary, it was defined as a fluid dram
         or one quarter of a tablespoonful.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

teaspoonful
    n 1: as much as a teaspoon will hold [syn: teaspoon,
         teaspoonful]