1 U.S. customary teaspoon = 1/6 U.S. fl. oz (4.93 ml)
1 Imperial teaspoon = 1/8 Imperial fl. oz (3.55 ml)
1 metric teaspoon = 5 ml (used in Australia, Canada, New Zealand and U.K.)
In a more general sense, a teaspoon is any small spoon, usually used to stir the contents of a cup of tea or coffee.
Note that a teaspoon used for stirring hot drinks is unlikely to contain the standard amount of material. For measuring precisely, special measuring spoons are available and should be used.
Other spoon measures include Tablespoon and Desertspoon. In all cases a spoon measure may be heaped or levelled.