Instinctive Reaction is a term used to describe an immediate, automatic response to a stimulus without conscious thought. It is often referred to as a 'GUT reaction' because it is based on instinct and intuition rather than rational thought. The term is often used in psychology to refer to the involuntary reactions that occur when faced with a situation or event. The crossword solution for this term is GUT, which is an acronym for 'GUT-level Unconscious Thinking'.