A rack of thirty beers.
"The bartender asked me to pay for a whole rack of thirty beers since I was hosting the party alone."
In plain English: Thirty is the number that comes right after twenty-nine and before thirty-one.
"The clock struck thirty when we decided to leave for the party."
The cardinal number occurring after twenty-nine and before thirty-one, represented in Arabic numerals as 30.
"Thirty people gathered at the meeting to discuss the new project plans."
The word thirty comes from Old English þrītiġ, which literally meant "three tens." It entered Middle English as a variation of thriti and eventually became the modern form we use today.