Literal translation: “sign”. It pertains to a mental sign, image, or vision that may arise in meditation. Usually pertains to a moment in concentration meditation (samatha jhana, samadhi) immediately preceding the reaching of the jhana state. Common nimittas are seeing the breath as a light, or cloud of smoke, or some other visual phenomenon.
I believe that the nimitta of seeing a light is the result of neurotransmitter release in the brain.
My quickest way to reach first jhana is to use the breath as the object and wait for the breath to become a light (this is the nimitta), then to focus upon both the light and the breath simultaneously at which point it becomes extremely bright and first jhana is reached.