Nimitta is used in two different ways:
One is a “sign” that comes to the mind as a kamma vipaka, especially close to the cuti-patisandhi moment when one is about to grasp a new bhava. That is the one discussed in the post.
The second is actually not in Buddha Dhamma, but was incorporated to Buddha Dhamma in the Visuddhimagga by Buddhaghosa. Hindu yogis did anariya kasina bhavana taking some object (ball of clay, dish of water, a light, etc) as the kasina object. This kasina object is also called a nimitta.
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