A mastermind is someone who plans and directs a complicated scheme or operation, especially one requiring intelligence and careful coordination. When detectives discover that a criminal mastermind organized an elaborate bank heist, they mean someone brilliant planned every detail: the timing, the escape routes, how to disable alarms, and how each person would play their part.

The word suggests exceptional intelligence used for complex planning. A student might be the mastermind behind an elaborate surprise party, coordinating friends to bring decorations, distract the guest of honor, and arrive at exactly the right moment. Scientists working together might have one researcher who serves as the mastermind of their project, seeing how all the pieces fit together.

While the word often appears in spy novels and heist movies describing villains, it simply means the strategic planner behind any complicated undertaking. You could mastermind a charity fundraiser, a school play, or a neighborhood cleanup. The word emphasizes the combination of intelligence, vision, and organizational skill needed to make something complex actually work. To mastermind something means to be its chief planner and coordinator, the person who sees the big picture and makes sure everyone knows their role.