To define a **motorcycle**, it is important to consider two key characteristics:

- **Number of wheels**: Motorcycles are **two-wheeled** vehicles.

- **Engine displacement and speed**: They must meet at least one of these two conditions:
- Have an engine with a **displacement greater than 50 cubic centimeters**.
- Reach a **maximum speed greater than 45 km/h**.

These characteristics distinguish motorcycles from other vehicles such as mopeds, which can have **two, three, or four wheels** and **less powerful** engines, with a lower maximum speed. Therefore, the correct definition of a motorcycle is one that includes two-wheeled vehicles with an engine displacement greater than 50 cubic centimeters and/or a maximum speed greater than 45 km/h.