Safety cannot be measured because it's a feeling. One person feels safe climbing a mountain without a rope and the next person is petrified. Safety is just word to describe a concept. It's different to the wavelength of light or force or charge. These things are based on fundamental properties of the universe that can be measured and are repeatable.
A reasonable approximation might be to consider the likelihood of an adverse event given a use case over time. We could say that an accident every million hours is our definition of safe but that is completely arbitrary in the way that the physical laws and constants are not.