In Mobile Suit Gundam: The Witch From Mercury, the title refers to Suletta Mercury, who is indeed referred to as a "witch" on numerous occasions. Sophie and Norea also specifically refer to themselves as "witches" during the attack on Plant Quetta.
At no point is it ever explained what a "witch" actually is within the context of the series. I can infer that it refers to
artificial humans created for the purpose of piloting Gundams, as Suletta, Sophie, and Norea all are IIRC,
but it's possible I've misunderstood or am missing something. So what exactly is a "witch"?