means A specialized segment of a market, audience, or environment where a particular thing or person fits perfectly — or, literally, a recessed nook in a wall.

from From French niche, meaning a recess or nook, which most likely traces back to the Old French verb nichier, 'to nest' — itself thought to descend from Latin nidus, 'a nest.' So the modern marketing buzzword still carries the warmth of its origin: a small, sheltered spot built exactly to someone's shape, like a bird's hollow tucked into a wall.