ABX discrimination is a term that is used for three stimuli presented on an ABX trial. The third is the focus. The first two stimuli (A and B) are standard, S1 and S2 in a randomly chosen order, and the subjects’ task is to choose which of the two is matched by the final stimulus (X).
Macmillan, N.A. & C.D. Creelman. 1991. Detection Theory: A User’s Guide. New York: Cambridge University Press.