The term Download Game describes a video game that is distributed via download instead of as a physical medium like a DVD or BluRay disc. The first Tomb Raider related game to be distributed in this way was Lara Croft and the Guardian of Light in 2012.
With the availability of high speed internet download games have become more and more popular as they are cheaper to produce and distribute. On the other hand these games can not be resold as with a disc.
A downloadable game should not be confused with Downloadable Content which is not playable as a stand alone game but plugs into a already purchased disk or download version and extends it with new features.