I stands for the Infrared Passband, the Near-infrared band nearest Visible Light in the UBV Photometric System. Centered around 806 nm, with a width (Full Width At Half Maximum) of 149 nm. I may also indicate a filter adhering to the specification.
The letter is also used to indicate the Magnitude of the signal observed through the filter, e.g., citing "I = 1.5".
UBV Photometric System