It’s a bicycle that features an auxiliary electric motor that helps the cyclist only when he’s actually pedalling. European Directive 2002/24/EC calls for three characteristics: electric motor with power not above 250W, power supply cut-off at a speed of 25 km/h and motor power supplied on only while the cyclist is pedalling.
If the motor of an e-bike runs also when the cyclist is not pedalling, it’s classified as an electric moped with a different set of standards. Tampering with electric bicycles exposes the user to serious criminal fines and civil liability profiles in the event of an accident.