english
1
general::
Also, that is to say. To explain more clearly, in other words, as in It's on the first floor, that is, at street level, or We're coming next month, that is to say, in November. [Early 1600s] Also see under THAT'S.
American Heritage Idioms
2
general::
Phrase(s): [that is] See the entries beginning with that’s.
McGrawhill's American Idioms And Phrasal Verbs