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What is a commonly used expression meaning "at every hour", e.g. 10:00, 11:00, 12:00 but not 10:18, 11:18, 12:18?
"Every hour" does not seem to distinguish between the two cases. Also it seems that "round hour" is not used similarly to "round number". And "every exact hour" does not seem to be commonly used either.
Where I'm from, you'd say
Every hour, on the hour
"On the hour" means "at no minutes past", but it wouldn't be clear alone that it happens every hour.
I could say to you, at 8.49 am, that "the train leaves on the hour", and you'd know that there was a train at 9.00 am, but not necessarily that there would be one at 10.00 am.
"On the hour" is possible, but is ultimately still ambiguous (Lexico: "At the same time every hour, or at the beginning of each hour").
"At the top of the hour" is good, although mostly an Americanism (M-W: "US: at the beginning of the hour (at 12:00, 1:00, 2:00, etc.)"). However, if you said "it happens at the top of the hour", people might think you were referring only to the next hour rather than each hour, so you might have to say either "it happens hourly, at the top of the hour" or "it happens at the top of each hour".
Other possibilities include:
