► The Chosen standard used for measuring a physical quantity
► Unit should be:
- Well Defined
- easy to reproduce
- easy to compare
- internationally accepted
- independent of changes in physical conditions
► Units are of two types- i. fundamental Unit ii. Derived Unit
In SI units , there are seven fundamental units given in the following table:
| Physical Quantity | SI Unit | Symbol |
| Length | metre | m |
| Mass | Kilogram | kg |
| Time | second | s |
| Electric Current | ampere | A |
| Temperature | kelvin | K |
| Luminuous intensity | candela | Cd |
| Amount of substance | mole | mol |
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