Deep Strike is a Universal Special Rule in Warhammer 40,000 representing the ability of some units to enter battle directly, as opposed to coming in from an edge. This generally involves dropping in from above, by such means as Drop Pods, grav-chutes (favored method of Elysian Drop Troops), or just flying in, though some units drill in from below (such as the Tyranid Mawloc or Imperial Guard's Hades Breaching Drill) and others teleport or otherwise warp in (like Space Marine Terminators and Chaos Daemons). It retains its existence in 8th Edition, but under different names ("Aid Unlooked For", "Teleport Strike", etc.). Based on the current 8th edition rules after the second Big FAQ, you can only Deep Strike starting with the second Battle Round. Units that were not deployed with a Deep Strike until the end of the third Battle Round count as destroyed.
The term seems to be used in-universe, or at least in the terminology of the Codex Astartes, as Commander Indrick Boreale describes Steel Rain as involving "multiple, simultaneous, and devastating defensive Deep Strikes."