1. Temporary loss of consciousness due to decreased blood flow to the brain.
2. Momentary loss of consciousness, as in absence.
3. Temporary loss of vision, without alteration of consciousness, due to positive g (gravity) forces; caused by temporary decreased blood flow in the central retinal artery, and seen mostly in aviators.
4. A transient episode that occurs during a state of intense intoxication (alcoholic blackout) of which the person has no recall, despite apparently having been conscious at the time.
The definition information for blackout is provided by Stedman's. You can search our medical dictionary here.