Online means you expect the responses to come quickly (seconds) after launching the request. The opposite is "offline" where you expect the results to come a long time after making the request (hours / days).

ClickHouse is designed so you can build dashboard with it. Other offline system are designed so you can build reports that you send in PDF over email with them.

Live and real-time

It is a rather old acronym. The other option was batch processing, you will get your results in the mail type thing.

Here "Online" means results while connected to the system, not real time since there is no time requirement for results.