"Acid" (unofficial name) is the hazard that appears only in the sixth and final Wily Castle stage in Mega Man 2. It appears as a large, red pool of liquid that drips from the ceiling. Many of these pools of liquid are seen all throughout the narrow passageways, and can damage Mega Man heavily if he makes contact with it, so the player must avoid these as much as possible.
Due to the drops being red in color, the acid drops seem to resemble blood.
Acid was present as a threat in the Mega Man cartoon show, where a spy sent by Dr. Wily traps Mega Man in the bathroom, and drowns him in acid in an attempt to dissolve him. This may either be a callback to the acid used in the games, or completely coincidental.