Phalanx
In the strictest sense, Phalanx is Greek for "finger" (in fact medically your finger bones are still called "Phalanxs"), but it is most well-known in the real world as a term for an ancient Greek military formation, in which spear-wielding warriors known as Hoplites 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
The Imperial Fists' fortress-monastery is the battle station Phalanx, a giant star fortress bigger than any Imperial vessel. Even the Furious Abyss Class, which were built as a combination of what-if engineering and fuck-you-mine-is-bigger oneupmanship, were small compared to it. There are two different accounts of how it came about - in one Rogal Dorn found it near Inwit and made it his by the time the Emperor found him and set him on the Great Crusade, whilst in the other he had it built himself. 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. Or at least, it was- at the moment it's being invaded by an army of Iron Warriors under the command of Warsmith Shon'tu and are just barely being held back by the combined forces of the Imperial Fists' 3rd Company and the Legion of the Damned. Which is some bullshit because the Phalanx was already invaded by a daemonic army led by the Daemon Prince Abraxes that could barely be held off by the might of the First Company and companies of Imperial Fist successor chapters. Invading when the loyalists are busy holding off Abaddon's latest Black Crusade kinda helps.