An Etymological Dictionary of Astronomy and Astrophysics

1. The required brightness for an object in order to be detected by an observing instrument.
   A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.
   

2. Of a radio receiver or similar device, the minimum input signal required to produce a specified output signal having a specified signal-to-noise ratio.

See also: State noun from → sensitive.