Robotic devices human-sized and up, featuring arms and legs as a prominent component of their design.
We've currently ton a ton of mecha tropes, but no actual generic mecha page, so here's my attempt to fix that. I'm finding that it's actually rather difficult to define mecha succinctly-but-accurately.
A mecha is a robotic machine, which can vary widely in size, configuration, and purpose; the term covers everything from man-sized Powered Armor to skyscraper-sized (or larger!) Humongous Mecha and all points in between. The only virtually universal aspect of mecha design is the inclusion of limbs. Mecha will have arms and/or legs where other types of machinery would have wheels, treads, wings, or other means of propulsion -- mecha may have these things as well, but they'll have them in addition to their limbs. These limbs may or may not follow a humanoid configuration (two arms and two legs), but they'll typically be used how you'd expect them to; legs primarily for locomotion, and arms for interacting with objects and the surrounding environment. (Mecha have Arm Cannons as often as not, though, so such interaction may be limited to "blowing it to pieces".) Mecha are usually vehicles, controlled from an onboard cockpit by a single pilot or small crew. Exceptions exist, however, such mecha that is controlled externally (by voice commands, motion capture, or other means), a mecha-style legged Base on Wheels (with an on-board crew of dozens or more) or fully autonomous mecha (which have no need for crew at all). The dividing line between an autonomous mecha and a Robot is somewhat ambiguous, though generally speaking, a robot is human-sized and an autonomous mecha is really huge. The term "mecha" (which is both singular and plural, like "fish") is originally of Japanese origin, derived from the English word "mechanism", where it refers to machinery and vehicles in general. English-speakers, however, typically use the narrower definition presented here. Both Japan and the West have a long history of mecha in fiction, though they tend to have different styles. The classic Eastern-style mecha is the Humongous Mecha; essentially giant infantry, they're usually humanoid, wield hand-held weapons like a human soldier, and are relatively light on their feet. The typical Western-style mecha, on the other hand, is a Walking Tank; nonhumanoid, with fixed or turreted weapons, and favoring heavy armor over nimbleness. Mecha is an incredibly broad category, with numerous subdivisions. In general, mecha are grouped into one of two categories: Super Robot, where mecha are treated like Superheroes, or Real Robot, where mecha are treated like advanced military technology. In addition to those general categories, there are a large number of mecha subtropes based on configuration or abilities: