Sharpness in an AC circuit is measured by the Quality Factor.

Quality Factor:
The quality factor or Q-factor is defined as the ratio of the energy stored in a system to the energy dissipated in the system per cycle of oscillation. It is a dimensionless quantity and is represented by the symbol Q.

Mathematically, Q-factor is given by the ratio of the resonant frequency to the bandwidth of the circuit.

Q-factor = Resonant frequency / Bandwidth

The higher the Q-factor, the sharper the resonance peak of the circuit. A high Q-factor means that the circuit can store energy efficiently and can oscillate for a longer time without losing energy.

Sharpness in an AC Circuit:
In an AC circuit, the sharpness is determined by the Q-factor of the circuit. The Q-factor of the circuit can be defined as the ratio of the energy stored in the circuit to the energy lost per cycle of oscillation. The sharpness of the circuit is directly proportional to the Q-factor of the circuit.

In an AC circuit, the sharpness is determined by the amount of energy stored in the circuit and the amount of energy lost per cycle of oscillation. The more energy the circuit can store, the sharper the resonance peak of the circuit. The less energy the circuit loses, the sharper the resonance peak of the circuit.

Thus, the quality factor is the measure of sharpness in an AC circuit.