Mu (Μ μ) is the 12th letter of the Greek alphabet and has a numeric value of 40.
The lower-case letter μ is used as the symbol for:
- The SI prefix micro-, which represents one millionth, or 10-6.
- Formerly, the micron, an old unit corresponding to the micrometer.
- Rarely, to distinguish between one item represented by m and another (but m might be an easier option).
- The coefficient of friction, ie. the ratio of the limiting frictional force between two surfaces against the normal contact force.