ILR stands for Independent Local Radio, a term for any local independent station in the UK with a full-term licence.
Other types of legal independent radio in the UK include IRR (Independent Regional Radio) and INR (Independent National Radio). In addition, a number of commercial independent radio stations have had RSL licences, especially prior to obtaining full-time licences.
The early days
The first ILR's were licenced in the early 1970's. Each ILR had both FM and MW frequencies. Most ILR's served a single large town and its surrounding areas, but some served more than one town with more than one FM frequency - eg Radio Hallam, which initially served Sheffield and Rotherham (with extra transmitters added later for Doncaster and Barnsley).
Well into the 1980's, each town had only one ILR - except London, which had the news station LBC and the pop music station Capital Radio.
The late 1980s onwards
In the late 1980's, there was a sudden expansion in the number of ILRs for two reasons.
The first was the licensing of extra stations in large cities and towns (eg Kiss FM, Jazz FM and Melody FM in London, Buzz FM in Birmingham) and in towns previously considered to have been covered by other ILRs (eg KFM Radio in Stockport).
The second was the splitting of programmes on the MW and FM transmitters. In most areas, the original ILR continued to broadcast as normal on the FM frequencies, while opening a separate service - generally playing oldies - on MW.
In addition, Independent National Radio started in the late 1980s and early 1990's. Classic FM started broadcasting when the emergency services stopped using 98 to 102 MHz for 2-way communications, allowing the frequencies to be used for broadcasting. Soon afterwards, Virgin Radio and Talk Radio UK (now Talk Sport) took over the MW frequencies previously used by BBC Radio 3 and Radio 1 - which became FM only.
Another large expansion in independent radio came in the late 1990's, when the FM frequencies from 105 to 108 MHz were freed for broadcasting. (Previously they had been used for two-way radio communications). A substantial number of ILRs became licenced in small towns, and Independent Regional Radio (IRR) stations began covering entire regions from high-powered transmitters on high masts (such as Emley Moor in Yorkshire).
Into the future ?
The number of new stations on FM and MW has almost dried up, due to a lack of clear frequencies.
A substantial number of new stations have been created on DAB (Digital Audio Broadcasting) radio, but these are by nature regional and do not tend to have a local character.
Eventually (it may take years or it may take decades), it is probable that DAB receivers will become as commonplace as FM receivers are now. When that happens, it is possible that a number of stations currently broadcasting on FM will switch to DAB-only broadcasting. If and when this happens, a substantial number of frequencies may be freed for a new round of ILR stations to be licenced.