What is the distance between a lens and its principal focus point known as?

The distance between a lens and its principal focus point is known as the focal length. This term defines an important characteristic of a lens and is fundamental in optics because it determines how much the lens converges or diverges light. For converging lenses (like convex lenses), the focal length represents the distance from the lens where parallel rays of light will converge to a point. For diverging lenses (like concave lenses), it indicates the point where the light appears to diverge from after passing through the lens. Understanding the focal length is crucial for applications such as photography, designing optical instruments, and understanding vision correction. The other terms do not specifically define the distance to the principal focus or are related to different concepts in optics.