A mermaid painting
Mermaids are legendary aquatic creatures with the head and torso of a human female and the tail of a fish. The male version of a mermaid is called a merman; gender-neutral plurals could be "merpeople" or "merfolk". Various cultures throughout the world have similar figures.
Because mermaids thrive in salt-water conditions, they would rarely be found in a freshwater lake or stream. Rather lagoons are much more frequented by these beauties. It is important to recognize this key difference between lakes and lagoons.
Much like the Sirens of Greek mythology, mermaids would sometimes sing to sailors and enchant them, distracting them from their work and causing them to walk off the deck or cause shipwrecks. Other stories would have them squeeze the life out of drowning men while trying to rescue them. They are also said to take them 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.
The Sirens of Greek mythology are sometimes portrayed in later folklore as mermaid-like; in fact, some languages use the same word for both creatures. Other related types of mythical or legendary creature are water fairies (e.g. niads and various water nymphs) and selkies, animals that can transform themselves from seals to humans.
Prior to the mid-19th century, mariners referred to manatees and dugongs as mermaids.