The word is a compound of mere, the Old English word for "sea", and maid, a woman. The male equivalent is a merman.
The first mermaid was actually a man. He was the god Oannes from Babylon. He is often shown as a man with a fish's tail. According to written descriptions he had the head and body of a fish but under the fish head is a human head and under the tail are human feet.
Originally the sirens, often depicted as mermaids, were bird-women. This is the source of their beautiful singing voices.
A two-tailed mermaid is the basis for the Starbuck's logo. It has been somewhat edited because some people found the original image too sexually suggestive.
Two important mermaid facts are the symbols of the comb and mirror, which come from the goddess Venus. The mirror is so closely associated with Venus that the symbol for the planet Venus is a mirror (the circle with the "+" under it).
In heraldry the two-tailed mermaid is shown full face with the ends of her tails held in each hand. Both single-tailed and double-tailed varieties symbolize eloquence. If she has her comb and mirror with her then it means vanity.
For a brief period during the Middle Ages mer-ladies with wings were carved on tombstones. These are probably descendents of the bird-sirens used in classical times. They are meant to represent both mourners and guides bringing the spirit of the dead to the afterlife.
Mermaids were noted in British folklore as unlucky omens – both foretelling disaster and provoking it.
Some mermaids were described as monstrous in size, up to 2,000 feet
Mermaids sometimes sing to people and gods and enchant them, distracting them from their work and causing them to walk off the deck or run their ships aground. Other stories depict them squeezing the life out of drowning men while attempting to rescue them.
Mermaids are said to carry humans down to their underwater kingdoms. In Hans Christian Andersen's "The Little Mermaid", it is said that they forget that humans cannot breathe underwater, while others say they drown men out of spite.