Deep Strike

From Lexicanum

Deep Strike is used to denote the way certain troops can arrive in battle. It can be achieved via various means. Space Marines for example, can use drop pods or have their Terminators enter battle via teleportation (which becomes more reliable when using a Teleport Homer). Teleportation or drop pod assault are just two means of deep striking. Other armies might use tunnelling, specialised drop troops (the Imperial Guard for instance uses Elysian Drop Troops for this), jump packs or jetpacks, or some other technique to achieve this.
A deep strike may be used to great effect for dropping powerfully equipped troops directly into the enemy lines. This is extremely effective for anti-armor units with melta weapons or those with similar effect, as it allows them to close quickly into striking distance and use the short range guns. Deep striking is also a way for an entire army to deploy in some scenarios, if the army is rapidly deploying into the area or deploying straight from dropships.

There is a downside to deep striking. Deep striking units are inaccurate when landing, so two separate units may not be able to function together as planned. This inaccuracy also creates the potential for destructive side effects. This may include landing on terrain that is dangerous, such as lava, or even landing in an area completely different from that which was the target. There are ways to counter this, however, especially through the use of some sort of signaling device, such as a Space Marine teleport homer.

Sources

• Warhammer 40,000 Rulebook, edition IV, p84