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A teaspoon, as an item of cutlery, is a small spoon, commonly part of a silverware (usually silver plated, German silver or now, stainless steel) place setting, suitable for stirring and sipping the contents of a cup of tea or coffee.
A high school project estimated the average weight of a conventional teaspoon made of metal to be approximately 25 grams. Teaspoons with longer handles, such as iced tea spoons, are commonly used also for ice cream desserts or floats. Similar spoons include the tablespoon and the dessert spoon, the latter intermediate in size between a teaspoon and a tablespoon, used in eating dessert and sometimes soup or cereals. Much less common is the coffee spoon, which is a smaller version of the teaspoon. Another teaspoon, called an orange spoon (in American English: grapefruit spoon), tapers to a sharp point or teeth, and is used to separate citrus fruits from their membranes. A bar spoon, equivalent to a teaspoon, is used in measuring ingredients for mixed drinks.
A container designed to hold extra teaspoons, called a spooner, usually in a set with a covered sugar container, formed a part of Victorian table service.
Measure of volume 
In some countries, a teaspoon full (also "teaspoonful" or simply "teaspoon") is a unit of volume, especially in cooking recipes and pharmaceutic prescriptions. In English it is abbreviated as t., ts., tsp. or tspn. and never capitalized, as capitals are customarily reserved for the larger tablespoon ("T." or "Tbls." or "Tb."). In German and Dutch teaspoon is abbreviated TL, for Teelöffel and Theelepel respectively.
As an unofficial but once widely used unit of Apothecaries' measure, the teaspoon is equal to 1 fluid dram (or drachm) and thus 1⁄4 of a tablespoon or 1⁄8 of a fluid ounce. The Apothecaries' teaspoon (formerly tea spoon or tea-spoon) was formally known by the Latin cochleare minus, abbreviated cochl. min. to distinguish it from the tablespoon or cochleare majus (cochl. maj.).
When tea-drinking was first introduced to England circa 1660, tea was rare and expensive, as a consequence of which teacups and teaspoons were smaller than today. This situation persisted until about 1710, when the East India Company began importing tea directly from China. As the price of tea declined, the size of teacups and teaspoons increased. By the 1730s, the teaspoon as a unit of culinary measure had increased to 1⁄3 of a tablespoon, but the apothecary unit of measure remained the same. Nevertheless, the teaspoon, usually under its Latin name, continued to be used in Apothecaries' measures for several more decades, with the original definition of one fluid dram.
In the United States one teaspoon as a unit of culinary measure is 1⁄3 tablespoon, that is, ~4.93 mL; it is exactly 11⁄3 U.S. fl. drams, 1⁄6 U.S. fl. oz, 1⁄48 cup, and 1⁄768 U.S. liquid gallon (see United States customary units for relative volumes of these other measures) and approximately 1⁄3 cubic inch.
Common teaspoons are not designed to contain a standard volume. In practice, they may hold anything between 2.5 mL and 6 mL of liquid, so such spoons are not suitable for precise measurements, in particular for medicine.
If a recipe calls for a level teaspoonful of a dry ingredient (salt, flour, etc.), this refers to an approximately levelled filling of the spoon, producing the same volume as for liquids.
A rounded teaspoonful is a larger but less precise measure, produced without levelling the ingredient off nor heaping it as high as possible.
A heaping (American English) or heaped (British English) teaspoonful is a larger inexact measure, equal to the most that can be obtained by scooping the dry ingredient up without levelling it off. For some ingredients, e.g. flour, this quantity can vary considerably.
When no particular type of teaspoonful is specified for a dry ingredient it may mean a level or a rounded spoonful, never a heaping/heaped one.
See also 
- Caddy spoon, a specialized spoon used for taking dried tea out of a storage container
- Cooking weights and measures
- Average weight of a conventional teaspoon made of metal. Wikiversity, peer review verified 2012.
- "ˈtea-spoon, n.". OED Online. Oxford University Press. 27 June 2012
- London Gazette Issue 2203 27 December 1686 page 2 "three small gilded Tea Spoons"
- Robert Borneman Ludy (1907). Answers to questions prescribed by pharmaceutical state boards. J.J. McVey. p. 125. Retrieved 19 December 2011.
- Dr. Collins (1803). Practical rules for the management and medical treatment of Negro slaves in the sugar colonies. Printed by J. Barfield, for Vernor and Hood. p. 465. Retrieved 19 December 2011.
- Popular encyclopedia (1884). The popular encyclopedia; or, 'Conversations Lexicon': [ed. by A. Whitelaw from the Encyclopedia Americana].. p. 11. Retrieved 19 December 2011.
- Henri Milne-Edwards; Pierre Henri L.D. Vavasseur (1831). A manual of materia medica and pharmacy, from the Fr. of H.M. Edwards and P. Vavasseur, corrected by J. Davies. p. 12. Retrieved 19 December 2011.
- Robert Eglesfeld Griffith (1859). A universal formulary: containing the methods of preparing and administering officinal and other medicines. The whole adapted to physicians and pharmaceutists. H.C. Lea. p. 25. Retrieved 19 December 2011.
- Beringer, Paul (19 May 2005). Remington: The Science And Practice Of Pharmacy. Lippincott Williams & Wilkins. p. 110. ISBN 978-0-7817-4673-1. Retrieved 27 June 2012.
- 21CFR101.9(b)(5)(viii) 21CFR101.9