The bandwidth of an oscilloscope is:

The bandwidth of an oscilloscope is the primary specification that dictates its ability to accurately measure and display high-frequency signals. It refers to the maximum frequency at which an applied sine wave is displayed at an amplitude of at least 70.7% (-3 dB) of its actual value. In simpler terms for exam purposes, bandwidth indicates the highest frequency signal the scope can reliably display. If you try to display a signal higher than the scope's bandwidth, the signal's amplitude will appear significantly attenuated, and fast-changing details like square wave edges will become distorted. Distractors like 'gain compression' or 'time-base accuracy' relate to other RF or instrument characteristics but do not define the upper frequency limit of the oscilloscope's vertical amplifiers.