A mermaid is a legendary creature with a female human head and torso (if it's male, it's called a merman) and the tail of a fish, which inhabits the water. Some sailors claim to have seen mermaids; what they actually saw are probably manatees. A mermaid-like creature having two tails is a melusine.
In heraldry the charge of a mermaid is commonly represented with a comb and a mirror, and blazoned as a 'mermaid in her vanity.' Mermaids are fascinating creatures because they are one of the few who have formed their own civilizations (in folklore.)
Mermaids are present in many movies, books, etc., and have become one of the most popular creatures of Pop Culture. The most popular mermaid is probably that one in Hans Christian Andersen's The Little Mermaid, which was later turned into a Disney movie. Another popular movie to feature a mermaid was Splash, starring Daryl Hannah. Mermaids and Mermen (called Merpeople) are present in the Harry Potter series, specifically in Harry Potter and the Goblet of Fire.