A rigorous test or appraisal of the quality or worth of something.
In the 1960s, a party based around the use and advocacy of the psychedelic drug LSD ("acid").
A test in which a person is exposed to flames in order to assess their truthfulness, commitment, courage etc.
A situation in which a soldier or other combatant faces the discharge of opposing weapons, as a test of their fortitude.
Any ordeal which tests one's strength, endurance or resolve.