What is the name of the beam weapon Samus wields?

The beam weapon Samus wields is her Arm Cannon. Built into the Power Suit, it lets her fire energy beams, charge shots, and—after upgrades—specialized attacks and missiles.

The Arm Cannon is more than a simple laser gun. Across the series, Samus upgrades it with weapons such as the Wave Beam, Ice Beam, Plasma Beam, and various Charge Beam enhancements. In Metroid Prime, it also supports several distinct beam types and can be used to scan objects through her visor.

A common mix-up is naming one of those upgrades as the weapon itself. Plasma Beam and Ice Beam are firing modes or upgrades; the Arm Cannon is the mounted weapon that carries and projects them. Its design is one of the defining visual features of Samus and her Power Suit.