Lots of different ways. For example, the legal definition of the U.S. standard units (foot, gallon, pound, etc.) are in terms of SI units.
The principal SI units used to derive all other SI units are the base SI units. These are the units for physical quantities such as length, time, mass, electric current, temperature, amount of substance, and luminous intensity.
When SI units combine, they create derived SI units that are used to express physical quantities. These derived units are formed by multiplying or dividing the base SI units. Examples include the Newton (kg*m/s^2) for force and the Pascal (N/m^2) for pressure.
A system of measurements. SI is the most used world-wide now (Systeme Internationale). The old British system using feet and pounds is now largely superseded though it is still used in some more traditional industry, especially in the US.
There are different units for measuring different attributes.
SI units