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.
Among its uses is the observation of NIR from protoplanetary disks associated with young stellar objects.
The letter is also used to indicate the magnitude of the signal observed through the filter, e.g., citing "I = 1.5".