Two figures are said to be similar when all corresponding angles are equal and all distances are increased (or decreased) in the same ratio, called
the ratio of magnification (Coxeter and
Greitzer 1967, p. 94). A transformation that takes figures to similar figures
is called a similarity.

Two figures are directly similar when all corresponding angles are equal and described in the same rotational sense.
This relationship is written .
(The symbol
is also used to mean "is the same order of magnitude as" and "is asymptotic to.") Two figures are inversely
similar when all corresponding angles are equal and
described in the opposite rotational sense.