What does T5 mean?

I see it mentioned when refering to lighting, but its not in the https://www.rollitup.org/newbie-central/412440-vocabulary-slang-terms-updated-missing.html thread

It's a linear fluorescent bulb type. T5, T8, and T12 refer to the diameter of the bulb.

I think it has something to do with tube diameter?

[h=5]What are T5 lamps?[/h] T5 lamps are fluorescent lamps that are 5/8" of an inch in diameter. This report discusses only linear T5 lamps. Differences in length and socket pin design versus conventional fluorescent lamps prevent any problems with electric circuits or human factors. This section focuses on the physical characteristics of T5 systems compared with T8 systems.
[h=5]What does T5 mean?[/h] The "T" in lamp nomenclature represents the shape of the lamp-tubular. The number following the "T" usually represents the diameter of the lamp in eighths of an inch (1 inch equals 2.5 centimeters). T5 lamps have a diameter equal to 5 times an eighth of an inch, or 5/8". These lamps are approximately 40% smaller than T8 lamps, which are one inch in diameter, and almost 60% smaller than T12 lamps, which are 1½" in diameter. Figure 1 shows diagrams of lamp ends of T5, T8, and T12 lamps. Figure 1 also shows that pin base type of T5 lamps is different from that of T8 and T12 lamps. T5 lamps have a miniature bi-pin base while T8 and T12 lamps use a medium bi-pin base.

T5 is a floresent light. Small 1/2 inch tubes. Much like the ones in offices. But those are T12.