"Symmetry" is a recognition of the matching-ness
of the parts of a shape.
For instance, the human body is said to have
"bilateral" (two-sided) symmetry, because the left and right halves of the body
mirror each other, as you can see in the image to the left.
But this symmetry applies only to the basic
framing structure of our bodies; much of our insides, our organs, don't match, as you can
see in the image to the right: watch the liver (brown) and the stomach (pink) change sides
when the organs are flipped.:
The
dotted line running down the middle of the left-hand graphic above is called "the axis of
symmetry".
If
you think of the bilateral-symmetry picture as being drawn on a sheet of transparent plastic
with a shish-kebob skewer punched through the dotted line, you could twirl the skewer (and
thus the sheet) 180° around and end up looking at the same picture, but from the other
side of the plastic sheet:
(Note:
If this animation experiences occasional breaks, you're probably viewing in Internet Explorer.
You might want to upgrade to a more capable browser.)
A figure
has as many symmetries as its plastic sheet can have skewers.
For
instance, a rectangle can be twirled through its middles, so a rectangle has two symmetries.
A
square can also be twirled along its diagonals, so a square has four symmetries.
A triangle
might have one or three lines of symmetry, but usually has none:
one line of symmetry
three lines of symmetry
no lines of symmetry
...and
a circle has infinitely-many lines of symmetry, since any line through its center (any diameter)
is also an axis of symmetry.
