Thin lenses — Define

What is meant by the terms focal length, principal axis and principal focus (focal point)?

The principal axis is the straight line passing through the centre of a lens, perpendicular to the lens. The principal focus (focal point) is the point on the principal axis where rays travelling parallel to the axis converge, for a converging lens, or appear to diverge from, for a diverging lens, after refraction. The focal length is the distance from the centre of the lens to the principal focus.