Answer: reactive thinking that relies heavily on situational cues, salient memories, and heuristic shortcuts to arrive quickly and confidently at judments.
Explanation: In "Thinking, Fast and Slow" by Daniel Kahneman, the author states that there are two modes of thinking System-1 which is instantaneous and is driven by instinct and System-2 which is slower and driven by logic. The definition of System-1 thinking is reactive thinking that relies heavily on situational cues, salient memories, and heuristic shortcuts to arrive quickly and confidently at judgments.