We can assume you have already looked this up in a couple of online dictionaries and found the answers wanting. The term has several meanings in different contexts, so - without knowing the context in which you speak - we will have a rough time answering this. Why not give us one of your definitions and tell us where you have trouble with it.
The obvious formality, anyway, just to get such trumpeted heralding out of the way. Popularly associated with bromide memes like "the whole is greater than the sum of its parts". The term holism was introduced by Jan Christian Smuts. This hypernymic idea has acquired several sub-categories and related offshoots since then, though emergentism arguably preceded it. It's etymologically descended from Greek holos (whole, complete, entire) + -ism (Greek, ismos; Latin, ismus: belief in, practice of, condition of, process). "Holistic" is an adjectival modification.
