The size of the electron spot on the face of a cathode ray tube. Note: The spot size is larger than the diameter of the electron beam because of the spillover of electrons into adjacent areas of the screen near the spot. The spot size is a function of the ability of the tube to focus the electron beam, as well as of the electron gun aperture. [From Weik ’89] 2. In facsimile systems, the diameter of the scanning spot or the recording spot. [From Weik ’89] 3. In single-mode optical fibers, the effective core diameter.