The phrase "in substantial" refers to something that has a significant or considerable impact. Synonyms for this phrase include "significant", "considerable", "meaningful", "substantive", "important", and "valuable". These words can be used to describe a variety of things, such as the impact of a new policy or the contribution of a team member to a project. Using synonyms for "in substantial" can help add variety and depth to your writing, as well as emphasize the importance of certain ideas or concepts. It is important to choose the right synonyms that best suit the context and intended meaning of the sentence or passage.