A safety case is a structured argument

Argument

In philosophy and logic, an argument is an attempt to persuade someone of something, or give evidence or reasons for accepting a particular conclusion.Argument may also refer to:-Mathematics and computer science:...

 that presents
evidence

Evidence

Evidence in its broadest sense includes everything that is used to determine or demonstrate the truth of an assertion. Giving or procuring evidence is the process of using those things that are either presumed to be true, or were themselves proven via evidence, to demonstrate an assertion's truth...

 that is intended to demonstrate that a system is
safe

System safety

The system safety concept calls for a risk management strategy based on identification, analysis of hazards and application of remedial controls using a systems-based approach...

. More specifically, a safety case aims to show
that specific safety claims are met.

A commonly cited definition is that given by UK Defence Standard
00-56 Issue 4:

"A Safety Case is a structured argument, supported by a body of
evidence, that provides a compelling, comprehensible and valid case
that a system is safe for a given application in a given environment".

Such an evidence-based approach can be contrasted with the more common
prescriptive approaches to safety commonly enforced by safety
standards, which require products to be developed using a prescribed
process. Such standards typically do not explicitly require evidence
of safety and instead rest on the assumption that following a "good"
process will result in a "good" product.

Railway safety case

A railway safety case is a safety case developed specifically for railways.