This thread does justice to a question put forth online several times and, as far as I can tell is only answered in part. I believe this question warrants a distinct and succinct answer. What I'm finding online is summarized below, and as one can see... there is something missing. I've been thinking about the primary similarities and differences between the relation, function and operation. For instance, a function is always a relation, but a relation is not necessarily a function. A relation is not necessarily a function because a relation, unlike a function, may involve more than one output. In the same vein it can also be said that an operation is always a function, but a function is not necessarily an operation. A function is not necessarily an operation because a function, unlike an operation, ______________________________. When is an operation not a function?