Is as simple as the value of which something will ultimately fail divided by the designed value (the "designed value" being the worst case to be expected in normal operation). They are surprisingly quite low in aerospace....lower than buildings or bridges for example.

So a simple example is if a turbine component will catastrophically fail at 750C but the max designed operating temperature under normal conditions is 250C then your safety factor is 3.

So any parameters can be substituted to calculate - the best example is the wing structural testing to failure always seen when new aircraft are designed. The wing is bent to a huge extreme by hydraulic jacks until it finally snaps. The load it finally snaps at is divided by the worst case normal operation load - and you have your safety factor