Magnification
The ratio of image height to object height, or ratio of image distance to object distance in an optical system.
Governing formula
m = h_image / h_object = -v / u
SI unit
Dimensionless
In depth
A positive magnification ($m > 0$) indicates an upright virtual image; a negative magnification ($m < 0$) indicates an inverted real image. Absolute value $|m| > 1$ represents enlargement.
Examples in the real world
A magnifying glass creating a $3×$ enlarged upright virtual image of small text.