What term describes a vacuum tube with a cathode, a single grid and a plate?

A vacuum tube with exactly three active electrodes—a cathode, a single control grid, and a plate—is called a "triode". The prefix "tri-" corresponds directly to these three functional elements inside the tube's envelope. The naming convention for standard vacuum tubes relies on counting these active elements. A "diode" has two (cathode and plate), a "tetrode" has four (adding a screen grid), and a "pentode" has five (adding a suppressor grid).