https://en.wikipedia.org/wiki/Focal_length

 https://photographylife.com/what-is-focal-length-in-photography

The focal length of an optical system is a measure of how strongly the system converges or diverges light.

Without getting into an in-depth physics discussion, the focal length of a lens is an optical property of the lens.
The exact definition is: Focal length measures the distance, in millimeters, between the “nodal point” of the lens and the camera’s sensor.