From the definition of angle, an angle is the figure formed by two rays (can be collinear or non-collinear) sharing a common endpoint, the rays are called the sides of the angle and the common endpoint is called the vertex of the angle.
The angle above is a typical example that contradict the definition stated in the question "an angle is two collinear rays with a common endpoint".
Because from the drawing the two rays that forms the angle are not collinear, which contradicts the definition in the question.
When two non-collinear rays share a common endpoint, it is also an angle.
From the drawing, the two rays that forms the angle are Ray BA and CA, and their common endpoint(Vertex) is at A.
The best definition is;
An angle is two rays with a common endpoint.
Or
An angle is the figure formed by two rays (can be collinear or non-collinear) sharing a common endpoint. The rays are called the sides of the angle and the common endpoint is called the vertex of the angle.
Also, Examples of undefined terms in geometry are point, line and plane.
Undefined terms can be combined together to form defined terms like angle.
