What classifies as a bike?

Can you elaborate on what constitutes a bike? Is it simply a two-wheeled vehicle designed for human propulsion, or does it encompass a broader spectrum of characteristics? For instance, does the presence of an engine disqualify a mode of transportation from being considered a bike, or are there electric bikes that still fit the criteria? Additionally, are there any specific design features or functional capabilities that are essential to the classification of a bike? Clarifying these points would help us better understand the boundaries of what qualifies as a bike.