Unit means part of something: a ~ in an army/air force, a navy. Component, building block of constructions and molecules.
It could mean a unit of measurement, ~ of currency (£$¥¤€).
Units sometimes means the digit in a number on the right, which is also called the "ones".
In exact sciences use is made of cgs/psi/cal and m kg s/N/J systems. The SI units in calculation and measurement prevail.
Some other ideas come from this word:
In the BBC TV series Doctor Who, UNIT is a military group on earth (UNified Intelligence Taskforce) who battle invading aliens.
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