two or more related parts
Many systems thinkers argue that a system requires a purpose in order to be defined. But POSIWID was a term developed to disabuse people of this faulty notion. If the purpose of a system is what it does, then all things are systems by definition. POSIWID has other important implications, of course, but one of them is that we can define the minimal viable system quite simply as, "two or more related parts."
Note: a "system" is very different in meaning than the term, "System (S)," which is used in DSRP Theory. These are two different uses of the same term. In DSRP, the term System means "the co-implication of the base-pair elements: part-whole"