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, resembling one large pincushion of death. The Phalanx was extremely powerful from the front but very vulnerable to attacks from the side, due to how difficult it is for the formation to move in any direction except forward, causing it to lose out to the more flexible Roman legionnaires.
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.
In Warhammer 40000
The Phalanx is a massive and ancient battle station that serves as the Fortress-Monastery of the Imperial Fists Space Marine Chapter. The size of a small moon, defended by layers of armors and shielding, and armed to the teeth with countless lance batteries, macrocannons, and torpedo launchers, the Phalanx is sure to ruin any Chaos Lord's day.
See it's own articlefor more details.