AIM (Alternative Investment Market) is a sub-market of the LSE (London Stock Exchange), which was launched on the 19th of June 1995. It is a replacement to the previous USM (Unlisted Securities Market) that was in operation since 1980.
The AIM allows smaller companies with low liquidity and demand/supply to be traded at a specific time of the day with a more flexible regulatory system than is applicable to the main market. This means that orders with instruments that are part of the AIM will be executed differently than the ones part of the main LSE market. Due to the low liquidity, it's possible that orders might not execute at the auctions for days or even weeks.
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.