Replacement fluorescent bulb.
F15T12/CW = Fluorescent, 15 Watts, Tubular, 8 eighths inch diameter, Cool White (4100 K)
Note that in metric designations, T26 (26 mm diameter) is equivalent to T8 (8/8" [1"] diameter).
In fluorescent tubes, for a given diameter, length correlates with watts. The longer the tube, the more Watts, and the more light output.
Fluorescent tube dimensions provided can be nominal, and may vary slightly within standardized min/max specifications.

Sold as a pair.