The term "key piece" is an idiomatic expression that refers to something essential or critical in a given situation. It can be used both literally and figuratively, depending on the context in which it's being used. In its literal sense, a key piece could refer to a physical object such as a key that unlocks a door or starts an engine. Figuratively speaking, a key piece is something that plays a crucial role in solving a problem or achieving a goal. For example, in a puzzle, the missing piece that completes the picture would be considered a key piece. Similarly, in a project at work, a particular team member's expertise could be referred to as a key piece in its success. In summary, "key piece" is an expression used to describe something that is vital and indispensable in achieving a desired outcome or resolving a problem.