Rays of light which are close to the principal axis of your curved mirror.

This is a common term because in a (cheaply produced) spherical mirror, incident parallel rays of light will only focus at the focal point if they are hitting the mirror close to the principal axis (ie. only if they are paraxial rays). Once you venture too far out from the principal axis they begin to miss the focal point and a caustic curve is formed.

On the other hand if you have a parabolic mirror the caustic curve is a thing of the past, and then anywhere a parallel ray hits the mirror, it will be reflected through the focal point!

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