Definition
Line symmetry
A figure has line symmetry when reflection in a line makes the figure match itself exactly.
What it means
Imagine the line as a mirror. Every point on one side has a matching point straight across the line at the same distance, and points on the line stay where they are. A figure may have no such line, one line, or several.
Remember it
Fold on the line: matching parts land exactly together.
Example
A rectangle that is not a square has two lines of symmetry: one vertical through its centre and one horizontal through its centre. Folding on either line makes the two halves coincide.
Watch for
Two sides that merely look similar do not prove line symmetry; the entire figure must match under reflection.
Common mix-up
The line of symmetry is the mirror line; line symmetry is the property the whole figure has.
Exact meaning
A figure has line symmetry if there is a line such that reflecting every point of the figure across that line maps the figure onto itself. Such a line is called a line of symmetry.