Definition
Line of reflection
A line of reflection is the mirror line that places each reflected point equally far across it.
What it means
Treat the line like a mirror or fold. From any point, travel straight across the line at a right angle. The reflected point lands the same distance on the other side. A point already on the line does not move.
Remember it
Straight across, same distance; points on the line stay.
Example
Point P is 3 squares left of a vertical line of reflection. Its image P′ is 3 squares right of the line on the same row. The segment PP′ crosses the line at a right angle, and the line cuts PP′ in half.
Watch for
The line of reflection does not have to pass through the shape. It can sit between a shape and its image, and it may be vertical, horizontal, or slanted.
Common mix-up
A line of symmetry is a special line of reflection that maps a whole figure onto itself. A reflection can instead move a figure to a separate mirror image.
Exact meaning
A line of reflection is the fixed mirror line for a reflection. Each point and its image lie on a segment perpendicular to the line, with the line halfway between them; a point on the line stays fixed.