A Custom Level is a level - or game, if it consists of multiple levels - that has been created by fans with the Tomb Raider Level Editor (TRLE) or the fan-made modification, the Next Generation Level Editor (NGLE). These levels use the TR4 engine (tomb4.exe), either in the original version that was part of the Level Editor, or in some modified or patched version to support a variety of enhancements.
Most Custom Levels only consist of a single level, but there have also been quite a few series with up to 15 Levels.
Despite the fact that the Level Editor was released a long time ago (together with Tomb Raider Chronicles in late 2000) there is still a very active Community of Level Builders that create new levels. There are now far over a thousand levels you can download from the internet. A comprehensive list would be too long for this Wiki.