Line of symmetry is a straight line, on a two dimensional graph or plane, dividing the curve, shape or the graph of an equation into two congruent halves. The line of symmetry is like a mirror: all points on one side of the line have one and only corresponding point on the opposite side of the line. Some shapes/graphs have more than one line of symmetry.
Examples:
y=3x2-12x+1 has a line of symmetry x=2.
y=|x-5| has a line of symmetry x=5.
4x2+9y2=49 has two lines of symmetry, x=0 and y=0.