In a paper which deals with the spectra of radio frequency cosmic events, the word “spectral turnover” is used. What is “spectral turnover”?
It just means, that when you look at the spectrum on a log-log plot, that it become flat or even reverses slope, in contrast to a steeper power law spectrum at higher and/or lower frequencies.
There are various physical reasons why a turnover might appear in the spectrum (e.g. a flattening in the underlying power-law distribution of electrons producing synchrotron emission or synchrotron self-absorption).
I was quickly able to find the following sketch (from this website) of a "GHz-peaked radio galaxy", which illustrates and explains the situation.