that is to say

Từ điển Oxford Advanced Learner 8th

▪ I. ˈthat is to say idiom
in other words
•three days from now, that is to say on Friday
Main entry: ↑say_idiom
 
▪ II. that is (to say) idiom
used to say what sth means or to give more information
• He's a local government administrator, that is to say a civil servant.
• You'll find her very helpful— if she's not too busy, that is.
Main entry: ↑that_idiom