What is meant by the term “critical frequency” at a given incidence angle?

In ionospheric propagation, the "critical frequency" refers to the absolute highest frequency of a radio wave that will be successfully refracted back to Earth at a specific angle of incidence. If a transmitted signal's frequency is higher than this critical limit, the ionosphere will not bend it sufficiently. Instead of returning to Earth, the signal will punch right through the ionosphere and be lost to space. The critical frequency depends on the ionization level of the ionosphere. Distractors mentioning signal-to-noise ratio or the lowest frequency describe completely different concepts, such as D-layer absorption limits or receiver performance.