Definition of that is (or that is to say) in US English:

that is (or that is to say)

phrase

  • Used to introduce or follow a clarification, interpretation, or correction of something already said.

    ‘androcentric—that is to say, male-dominated—concepts’
    ‘He was a long-haired kid with freckles. Last time I saw him, that is’
    • ‘He was a genius - that is to say, a man who does superlatively and without obvious effort something that most people cannot do by the uttermost exertion of their abilities.’
    • ‘Recent web typography articles stress that good typography requires a vertical grid, that is to say a solid vertical rhythm achieved with a consistent, measured line-height.’
    namely, that is to say, that is, to wit, to be specific, specifically, in other words, to put it another way
    in other words, to put it another way, to rephrase it
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