Following a suggestion by B. Clark (1997, priv. comm.) we use the following terminology. The numbers given here are for reference only, to make a somewhat confusing discussion more concrete, and are based on the current notional design of the MMA system.
``IF'' is a terribly confusing term, and we will avoid it whenever possible. Insofar as it must be used, an IF is a wire coming out of a receiver package. In the current design for the MMA, each wire carries an 8 GHz bandwidth. There is provision for switching halves of the IFs (that is, a 4 GHz bandwidth) independently, and each half is sometimes also called an IF. The current plan has a total of 16 GHz being sent down from each antenna to the correlator, for a total of 4 independent (4 GHz) IFs.
A baseband (BB) is the signal presented to a sampler. In the MMA case, each BB has a bandwidth of 2 GHz or less, and can be flexibly positioned within an IF band, or switched between IFs. The canonical design has 4 pairs of basebands, with the two basebands of each pair having the same frequency but different (linear) polarizations.
A channel is the resolution element of frequency, and is also referred to as a spectral point. In the canonical design each channel comes out of the correlator as an eight-byte complex number (four bytes real, four bytes imaginary).