unit
The word 'unité' in French is used in the same contexts as 'unit' in English. It can refer to a singular individual thing, a component of a larger group or system, a measure or standard of quantity, or a piece of equipment. In most cases, you can use 'unité' wherever you would use 'unit' in English.
The system uses the central processing unit to perform the operations.
In this sentence, 'unité' precedes 'centrale de traitement', which is often shortened to CPU in English. It refers to the primary component of a computer that performs most of the processing inside the computer.
An hour is the most commonly used unit of time.
In this instance, 'unité' is used to talk about measurements - an 'unité de temps' is a unit of time. The sentence discusses how an hour is a standard measure of time.