"a cup of tea"
Yes, but you cannot say "a tea" in English, so that's the point they try to make everyone notice here. If you say "un ceai", you can understand it as a "cup of tea". Just like Spanish, you can say "un té" and that means "una taza de té".
Actually you can say in English "A tea". You might have an exchange like: "Do you want a drink?" "Yes please!" "A tea or a coffee?" "Tea please."
Or someone might say: "Oooh, I could murder a tea right now!" or "I'm gasping for a tea!" The use of the words 'murder' and 'gasping' are figurative colloquialisms here, basically the person would really like a cup of tea.
It is the context that differentiates the usage from something like: "Lapsang Souchong is a tea originally from China." meaning it's a type of tea. In Romanian how would you say that?
Here I'm referring to English as spoken in the UK, the US or other countries may be different.
@AndreiChich I disagree, O cana de ceai literally translates to "A cup of tea" word by word as you read it left to right. So AbigailCar992519's translation is definitely an option
Don't quote me on this since I wasn't born in RO, but I'm pretty sure it would actually be, Un pahar cu ceai, or O cana de ceai, or something like that instead of "A tea/Un ceai" in RO when the English question is a cup of tea. The answers they give us are only partial to what "a cup of tea" is. (I speak RO very fluently fyi, just doing this to learn extra things or see if I'm missing any knowledge)