Structured Assurance Case is “a documented body of evidence that provides a convincing and valid argument that a specified set of critical claims regarding a system ’s properties are adequately justified for a given application in a given environment.1)” Much like a legal case presented in a courtroom, an assurance case is a comprehensive presentation of evidence and arguments that support claims about properties or behaviors of a product or system, such as, security, safety reliability, etc. Hence, the structured assurance case provides a structured, composable, and traceable model for demonstrating and verifying the plausibility and strength of claims made about a property of the product or system in question.
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4548534/#b13-v115.n03.a05