A telescope's collecting area is the area of the cross section of incoming EMR. For a small refracting telescope, this is generally the area of the aperture or the primary lens, i.e., the aperture's radius squared times π. For a reflector telescope, any part blocking the primary mirror e.g., a secondary mirror, or an instrument (if prime focus) would have to be subtracted to yield the collecting area. With a segmented mirror, there may be spaces between the segments that would be subtracted. The collecting area is related to how dim an object can be detected whereas the aperture is related to the angular resolution.
For arrays, the effective aperture may be larger (through aperture synthesis), providing better angular resolution if the individual units are spread out, but the collecting area remains the same whether they are close to each other or far apart. Some arrays are primarily aimed at better angular resolution, whereas others are aimed at sensitivity. The Square Kilometre Array is a planned array aimed at achieving a very large collecting area.
The term collecting area is used in the analogous sense for instruments and detectors that would not be termed telescopes and/or sense something other than EMR.