A single random event for which there are two and only two possible outcomes that are mutually exclusive and have a priori fixed (and complementary) probabilities of resulting. The trial is the realization of this process. Conventionally one outcome is termed a success and is assigned the score 1, the other is a failure and has the score zero. Thus the outcome might be 0 (no heads, one tail) or 1 (1 head, no tails).
Medical dictionary. 2011.