Nimitta

In Buddhism a Nimitta is a sign or mark by which objects are recognized. It refers to a general and relatively persistent characteristic in contrast to secondary characteristics. The apprehending of signs enables perception of objects and feeling tones, reinforcing ignorance and leading to suffering.

In addition, the word nimitta is used in Buddhist scriptures to refer to a causal ground (ex. to criticize someone), an auspicious sign or omen, or an outer appearance (ex. a reflection).

Animitta
Animitta, or the absence of signs, is one of the Three Gates of Liberation and a form of samadhi in Buddhism along with emptiness and wishlessness.

Perceiving with the senses without apprehending signs is central to the practice of sense restraint.

In Therevada
In Therevada commentary such as the Visuddhimagga, nimitta refers to mental images that arise during meditation. These images start unstable and vague (uggaha-nimitta) and grow progressively more vivid and stable as the mediator becomes more advanced. It is believed that when sufficiently stable (patibhāga-nimitta), the state of "access samadhi" is attained and cultivating the image will lead to Jhana.

It has been noted that earlier commentary such as the Vimuttimagga placed less emphasis on such visual nimitta than contemporary Therevada.

Nimitta in Therevada commentary can also refer to states of mind on the death bead based on past karma or believed to foretell the dying's future destiny.

In Yogacara
In Yogacara philosophy, nimitta refers to the delusion that signs apprehended by the mind correspond to real objects outside the mind. Nimitta-bhaga refers to the conception of something that is seen, the counterpart to the delusion of a self who sees.