1 there is only one basic substance or principle as the ground of reality,
b. (in epistemology) a theory that the object and datum of cognition are identical.
2. the reduction of all processes, structures, concepts, etc., to a single governing principle; the theoretical explanation of everything in terms of one principle.
3. the conception that there is one causal factor in history; the notion of a single element as primary determinant of behavior, social action, or institutional relations.