What's the definition of a "collection"?

Question

I cannot seem to find a formal definition for the following.

What's a "collection" in the context of set theory?

Answer 1

"Collections" have no formal existence in set theory. The word is deliberately left without a technical meaning, such that it is available for speaking about our intuitive non-rigorous idea about, erm, collections of things -- without implying that the collection we're talking about satisfies the formal conditions for being considered a "set" or "class", both of which are often technical terms.