For questions about the definition and usage of units of measurement.
Units of measurement are used to define the magnitude of a quantity, based on some agreed reference point. A physical quantity may, therefore, be written as the product of a number and a unit: $$Z = n \times [Z]$$
were $[Z]$ denotes the units of $Z$.
The most common units in scientific usage are those within the SI unit system, which defines seven base units, based on measurable physical quantities which are known to be invariant (with the exception of the kilogram, although this is soon to be remedied). All other SI units are then derived from this, using mathematical relationships between quantities.
This tag may be applied to questions which are about the definition of units and their usage. It may also be applied to the related concept of dimensional analysis, which allows the dimensions (and therefore units) of a quantity to be worked out based on a mathematical formula defining that quantity. For example: $$\text{speed} = \frac{\text{distance}}{\text{time}}$$
The dimensions of distance are length, which has units of metres in the SI system. The dimensions of time are time, which has units of seconds in the SI system. The dimensions of speed are therefore length divided by time, which has units of metres per second in the SI system. $$[\text{speed}] = \frac{[\text{length}]}{[\text{time}]}$$
