The phrase "that is" can be replaced by several synonyms depending on the context of use. The phrase can be substituted with words like 'namely,' 'to wit,' 'to be specific' or 'in other words.' These synonyms are perfect for use in academic or formal writing, and they add clarity and precision to the intended meaning. Additionally, other synonyms for "that is" include 'in essence,' 'basically,' 'fundamentally,' or 'intrinsically.' These synonyms can, in some cases, emphasize the point better than the original phrase. Overall, it is essential to understand the context of use before choosing which synonym works best.