The term R band is often used for a photometric system's
red band, a typical example being centered around 658 nm
with a full width at half maximum of 138 nm (a UBVRI photometric system specification).
R may indicate the band or 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 "R = 1.5".
Among other recent uses of the letter R for a passband are the
grizr band centered around 622 nm and the
SDSSugriz photometric systemr band centered around
6165 Å.
A non-astronomy-specific use of the term R band is for a radio
(microwave) band of 1.7 to 2.6 GHz, the term
used, for example, in the specification of equipment-parts, such
as waveguides.