The term "keystone man" refers to someone who is crucial or essential to the success of a particular project or endeavor. Some synonyms for this term include linchpin, cornerstone, fulcrum, pivot, and centerpiece. All these words denote a crucial piece that holds together or supports a larger structure. A linchpin, for example, is a small pin that holds a wheel in place and is essential for the machine to work. Similarly, a cornerstone is a vital part of a building's foundation, and the fulcrum is the point on which a lever rests, allowing it to exert force. Using synonyms is an effective way to enhance your writing, making it more varied and engaging.