The first time Nyquist Theorem was mentioned in class. It stated that we should sample at twice the highest frequency content of the signal. Example: If we wanted to sample $cos(2 \pi ft)$, the sampling frequency should be at least $2f$

However, in another course. The Nyquist Theorem was stated as such: the sampling frequency should be at least twice the bandwidth of the signal. Isn't the bandwidth of a single tone cosine 0? which makes the two definition contradictory.