14W T5 Black Light Black with Fixture 220V ( Firefly), 14W T-5 Fluorescent Tube Black Light UV-A

The T indicates that the shape of the bulb is tubular, and the last number (yy) is the diameter in eighths of an inch (sometimes in millimeters, rounded up to the nearest millimeter). Typical diameters are T12 or T38 (1+1⁄2 in or 38 mm) for magnetic or electronic ballasts, T8 or T26 (1 in or 25 mm) for smaller and often energy-saving lamps with magnetic or electronic ballasts, and T5 or T16 (5⁄8 in or 16 mm) for very small lamps, which may even operate from a battery-powered device.