A penny (pl. pence or pennies) is a coin or a unit of currency used in several English-speaking countries.
The penny is among the lowest denomination of coins in circulation.
In addition, variants of the word penny, with which they share a common root, are or were the names of certain units of currency in non-English-speaking countries:
In the United States 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.
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 penny" or "new pence" was changed to "one penny", "two pence" or "five pence", etc. 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 penny as a unit of currency dates back well over a thousand years, and for most of that period the silver penny was the principal denomination in circulation.
To "spend a penny" in British idiom means to urinate. The etymology of the phrase is literal; some public toilets used to be coin-operated, with a pre-decimal penny being the charge levied. Eventually, at around the same time as the introduction of decimal coinage, British Rail gradually introduced better public toilets with the name Superloo and the much higher charge of 6d.
Finding a penny is sometimes considered lucky and gives rise to the saying, "Find a penny, pick it up, and all the day: you'll have good luck." This may be a corruption of "See a pin and pick it up, all the day you'll have good luck" and similar verses, as quoted in The Frank C. Brown Collection of North Carolina Folklore and other sources.
It is commonly believed that the origin of the term "penny" in relation to nail size is based on the old custom in England of selling nails by the hundred. A hundred nails that sold for six pence were "six penny" nails. The larger the nail, the more a hundred nails would cost, hence the larger nails have a larger number for their penny size. This classification system was still used in England in the 18th century, but is obsolete there now.