Phalanx
In the strictest sense, Phalanx is Greek for "finger", but it is most well-known in the real world as a term for an ancient Greek military formation, in which spear-wielding warriors stood shoulder-to-shoulder and marched as a single rectangular block.
Necrons
The Necrons of Warhammer 40,000 use the term phalanx in a similar sense as the Greeks did, referring to a small formation (i.e. a squad or two, in game terms) of Necron Warriors, Immortals, or other foot-soldiers. The next size up would be a legion.
Imperial Fists
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The Imperial Fists' fortress-monastery is the battle station Phalanx, a giant star fortress bigger than any Imperial vessel. Apparently, Rogal Dorn had already found it near Inwit and made it his by the time the Emperor found him and set him on the Great Crusade. While Terra is the official homeworld of the Imperial Fists, in practice they were the first Fleet-based Chapter (okay, Legion, but we digress) with Phalanx serving as their primary training ground.