ASSERTION meaning: 1. a statement that you strongly believe is true: 2. a statement that you strongly believe is…. Learn more. ASSERTION definition: 1. a statement that you strongly believe is true: 2. a statement that you strongly believe is…. Learn more. An assertion is a declaration that's made emphatically , especially as part of an argument or as if it's to be understood as a statement of fact. Assertions are used to check design rules or specifications and generate warnings or errors in case of assertion failures.