a standard but unofficial name for the one-cent coin of the United States and Canada, worth 1/100 of the dollar: see penny (U.S. coin). Note that this word is not officially used by the United States Mint; they use cent.
In the USA and Canada, "penny" is normally used to refer to the coin; the quantity of money is a "cent". Elsewhere in the English-speaking world, the plural of "penny" is "pence" when referring to a quantity of money and "pennies" when referring to a number of coins. Thus a coin worth five times as much as one penny is worth five pence, but "five pennies" means five coins each of which is a penny.
In Canada, penny originally referred to pence coinage that they used until 1859 since there was a coin with the word "penny" on it (for pence). Since this was a term that they used for many years, the One Cent denomination stayed with the penny nickname ever since. The Royal Canadian Mint and the Federal Government of Canada don't officially call the One Cent coin a penny but a "One Cent" coin.
When dealing with British or Irish (pound) money, amounts of the decimal "new pence" less than £1 may be suffixed with "p", as in 2p, 5p, 26p, 72p. Pre-1971 amounts of less than 1/- (one shilling) were denoted with a "d" which derived from the term "denarius", as in 2d, 6d, 10d. The lettering "new pence" was changed to "pence" on British decimal coinage in 1982. Irish pound decimal coinage only used "p" to designate units (possibly as this sufficed for both the English word "pence", and Irish form "pingin").
The British decimal Twenty Pence (20p) coin was issued in June 1982 to fill in the obvious gap between the Ten Pence and Fifty Pencecoins; it rapidly gained common acceptance and is probably the most common of the current 'silver' British coinage.
The coin is minted from an alloy of 84% copper and 16% nickel (unlike the other 'silver' coins which are 75% copper, 25% nickel), weighs 5.00 grams and has a diameter of 21.4 millimetres.
Note that, like the fifty pence piece, the coin is not circular, but seven-sided to aid identification.