What is Alternative Investment Market?

Legal Definition
AIM (formerly the Alternative Investment Market) is a sub-market of the London Stock Exchange that was launched on 19 June 1995. It allows smaller, less-viable companies to float shares with a more flexible regulatory system than is applicable to the main market.

AIM has also started to become an international exchange, often due to its low regulatory burden, especially in relation to the U.S. Sarbanes–Oxley Act (though only a quarter of AIM-listed companies would qualify to list on a U.S. stock exchange even prior to passage of the Sarbanes–Oxley Act). As of December 2005 over 270 foreign companies had been admitted to the AIM.

The FTSE Group maintains three indices for measuring the AIM, which are the FTSE AIM UK 50 Index, FTSE AIM 100 Index, and FTSE AIM All-Share Index.
-- Wikipedia
Legal Definition
In the UK the market and stock of small companies that can't afford the LSE. Refer to over the counter market.