This semester teaching topology, I told the students that my "no holes" informal interpretation of compactness can be formalized as simply, chains of nonempty closed sets have nonempty intersection. (I also taught the open cover definition.)

I haven't seen this chain characterization in the textbooks I've used, but it's trivial to prove using a known result from infinitary order theory. (A poset is chain-complete iff it is directed-complete.) Surely, this has been done before. I just don't know where.

Rather than develop a bunch of order theory I didn't have time for in my topology class, I typed up an "elementary" proof, using "just" a well-ordering, that chain compactness is equivalent to the compactness in the usual sense.

If you know a thing or two about posets, you will recognize that the proof trivially generalizes into a proof that posets are chain-complete if and only if directed-complete. I don't know if my proof approach is new. But my recollection is that the standard proof uses induction on cardinality of the chains and directed sets, which I think is conceptually more elaborate than my approach of using a well-ordering to extract a minimal bad chain from a bad directed set.