The word unit comes from the Latin term units and designates the quality of what is unique or indivisible. Its meaning is what is considered individually and not plural.
Unit also means the union of components with a certain homogeneity or identity. A military unit is an organization made up of military personnel who belong to the same fraction of a military system. A hospital is made up of several units, the ICU (Intensive Care Unit) being one of the most common.
In Mathematics the unit is represented by the number one. It is the element from which all other natural numbers are built. Two is the successor of one unit (or sum of two units), three is the successor of two (or sum of three units), and so on.
In Sciences, the physical quantity that serves as a reference for a given measurement is designated as a measurement unit. For example, meter for length, kilogram for mass, second for time. The International System of Units aims to standardize measurement units. It is a system adopted in most countries around the world. The United States is an example of one of the few exceptions.