Definition Significant and substantial are two words that are often used interchangeably, but they actually have distinct meanings. Significant typically refers to something that is important, meaningful, or noteworthy. It implies that the subject in question has a considerable impact or influence. On the other hand, substantial usually refers to something that is large in size, amount, or extent. It suggests that the subject is considerable or significant in a physical sense. Impact When ... SUBSTANTIAL definition : 1. large in size, value, or importance: 2. relating to the main or most important things being…. Learn more. Something substantial is large in size, number, or amount: If you want to say someone spent a lot of money without being too specific, you could say they spent a substantial amount of money. Substantial means considerable in importance, value, degree, amount, or extent. It also means solidly built, strong, ample, or real. Find synonyms, antonyms, and examples of substantial in different contexts.