Synonyms for the phrase "spontaneous reaction" include words like unplanned, impulsive, instantaneous, spur-of-the-moment, and unpremeditated. These phrases all suggest a state of action or response that happens quickly and without much forethought or preparation. Other related terms might include instinctive, intuitive, reflexive, automatic, or natural, all of which imply that the reaction happens almost without conscious thought or effort. While spontaneity can be a valuable trait in some situations, it can also lead to rash or impulsive decisions, so it is important to balance this trait with careful consideration and thoughtful analysis.