A bachelor is a young or unmarried man; or bachelorette for a woman. Also, somewhat archaically, the holder of a bachelor's degree.
The educational term originates from the Latin bacca laurus, "decorated", whence more directly derives our adjective form baccalaureate. In the Middle Ages, under the influence of the Anglo-Saxon word bacheler (with an e), meaning a young person, the word "bachelor" came to denote a young man. (The conflation of terms was understandable, since in Classical times most students were male.)
The "bachelor lifestyle" became popular in the middle of the 20th century, particularly after World War II, when many men came home from Europe to find a shortage of suitable wives. The European-flavored, sexually forward, and "hard-playing" lifestyle often involved alcoholic drinks at a "bachelor pad" in the city, as depicted in Hollywood movies such as Boys' Night Out (see also playboy).