In the context of science fiction, it's a giant robot (often of humanoid aspect).
This may be to have a human pilot (or remote controlled by one), or simply have their own artificial intelligence.
Due to the humanoid aspect, mechas may also have a feminine appearance. Here are some examples:
Controversy: female mechas implies GTS theme?
The problem is that, despite having a feminine aspect, the metal composition and its robotic aspect take away some of its "human essence" (whatever that means), so some members of the GTS community don't consider female mechas as GTS theme.
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