The focal length is the distance from the lens (or mirror) to the: image object focal point center of curvature

Focal length is the distance of center of the lens from its FOCAL POINT

Explanation:

As we know that when straight beam of light incident on the lens which is parallel to the principle axis of the lens then this light will get refracted by the lens

After the refraction from the lens the light will pass through the a point on the principle axis. That common point is known as FOCAL POINT.

As as per lens formula we know that

now we have

so here we have

so focal point will be the point where image formed when straight parallel beam of light passes through it.