A reality check is a term used to describe an action or statement that brings someone back down to earth, reminding them of the actual state of things. It can refer to a situation where someone's beliefs, ideas, or expectations are challenged by something that happens in real life, causing them to re-evaluate their perception and adjust accordingly. In essence, it is a way of grounding oneself in reality after being caught up in fantasy, dreams, or idealized thoughts about how things should be.