The G13 is a bi-pin (2-pin) connector usually found at either end of specific fluorescent tubes. As a general rule the G13 can be found on T8 and T12 fluorescent tubes.
The ‘13’ suffix is derived roughly from the 12.7mm centre-to-centre distance between its 2 pins.
G13 bi-pin, G13 2-pin.
T8 fluorescent tube, T12 fluorescent tube, U-shaped tube.