LOL, or lol, is an initialism for laughing out loud and a popular element of Internet slang. It was first used almost exclusively on Usenet but has since become widespread in other forms of computer-mediated communication and even face-to-face communication. It is one of many initialisms for expressing bodily reactions, in particular laughter, as text, including initialisms for more emphatic expressions of laughter such as LMAO.

Source: Wikipedia