An Opponent Channel as well as a category of CVD where that opponent channel is affected. This includes Protan and Deutan defects. The Red-Green Opponent Channel is a reformulation of the Tristimulus values (LMS), which is a balance between the L-Cone (Red) and the M-Cone (Green).
Red-Green CVD accounts for 90%+ of all CVD (though this number may be incorrect according to the Tritan Epidemic). While red-green are among some of the Colors of Confusion, they are not the most noticeable ones.
See also Blue-Yellow.