A Mecha is a term commonly used for giant or otherwise very large robots. It can also be a shortened version of the English word "mechanical" or "machine". Mecha may also refer to a "Mechanical Organism".
Although they are mostly described as robots, mechas can incorporate some organic elements (acording to the Godzilla series). Examples of these are often refered to more as cyborgs. Two of the major examples in Godzilla series are Kiryu and Mecha-King Ghidorah, both of witch incorporate organic elements.
Most mechas in the Godzilla series and beyond are armed with an inconciveable amount of fire power. A prime example is Mechagodzilla 1, having toe, knee and finger missiles, an electric beam from his chest, twin maser beams from his eyes and a shield generator.
Most mechas are described as being radio controlled from an exterior force. Mechagodzilla 1 was controlled by the Simians, Mecha-King Ghidorah was used by the Futurians against the humans after the original's defeat at the hands of Godzilla, MG2 was controlled by humans as was M.O.G.U.E.R.A. Kiryu was, for the most part, under human control. However, when in contact with Godzilla he brakes free and goes on a rampage.