Why an amplifier can distort a perfectly clean signal at high frequency — even one well inside its rated bandwidth.
Slew rate is the maximum rate of change an amplifier's output voltage can physically achieve, specified in volts per microsecond (V/µs). It's a hard physical limit set by how much current is available internally to charge the amplifier's own compensation capacitance — and it has nothing to do with the amplifier's small-signal bandwidth or gain-bandwidth product. Ask an amplifier to move its output faster than its slew rate allows, and it simply can't. The output doesn't get quieter or lose a bit of gain — it changes shape entirely.