Fatigue is failure cause by cycling loads which do not exceed the yield stress of the material, typically over tens of thousands of cycles. Not to be confused with work hardening which occurs over relatively few cycles at stresses exceeding the yield stress of the material.
Fatigue strength is usually defined by the stress at which a material will exhibit no fatigue however many load cycles it is exposed to (aka endurance limit) or the load at which a sample will fail after N cycles. Also fatigue properties are a statistical distribution rather than an absolute value hence the qualification with 'mean', although this is often omitted as it is implicit in the definition.
In many cases low alloy steels have a fatigue strength which is close enough to their yield stress that once factors of safety have been taken into account it becomes a non-issue.
Fatigue limts can also be affected by the surface condition of the sample eg surface roughness, notches etc.