A compare-and-set request. E.g., I can read version 3 of a row, and write version 4 if the current version is still 3, so that if someone else writes version 4 I don't clobber whatever they just did.

That’s a really succinct definition. I like it. Thank you.

To be clear, "lightweight transactions" are a Cassandra specific term, nobody else uses that. Compare-and-set, atomic updates, or just transactions are the normal terms.

Pythian blog on the topic; note that Cassandra relies on Paxos as a consensus algorithm. Scylla will be using Raft: https://blog.pythian.com/lightweight-transactions-cassandra/