Noun

1. Acting automatically or involuntarily.
2. An action performed subconsciously, without any apparent direction from the mind; a thought which appears spontaneously in one's consciousness.
   • 1902, William James, The Varieties of Religious Experience, Folio Society 2008, p. 200:

     generalising this phenomenon, Mr Myers has given the name of automatism, sensory or motor, emotional or intellectual, to this whole sphere of effects, due to ‘uprushes’ into the ordinary consciousness of energies originating in the subliminal parts of the mind.

3. A surrealist painting technique whereby one attempts to move the brush, pen etc. without conscious control over it.