A deal breaker is a term used to describe something that is so important, critical or significant that it can either make or break an agreement, relationship, opportunity or situation. It refers to a condition or requirement that must be met for a transaction, negotiation or arrangement to proceed. If the "deal breaker" is not satisfied, then the parties involved may decide to terminate the deal, end the relationship or walk away from the opportunity. Essentially, it's an issue that is considered so essential and non-negotiable that if it isn’t met, everything else becomes irrelevant.