1. An individual, group, structure, or other entity regarded as an elementary structural or functional constituent of a whole.
2. A group regarded as a distinct entity within a larger group.
3. 1. A mechanical part or module.
   2. An entire apparatus or the equipment that performs a specific function.
4. A precisely specified quantity in terms of which the magnitudes of other quantities of the same kind can be stated.
5. Medicine. The quantity of a vaccine, serum, drug, or other agent necessary to produce a specific effect.
6. 1. A fixed amount of scholastic study used as a basis for calculating academic credits, usually measured in hours of classroom instruction or laboratory work.
   2. A section of an academic course focusing on a selected theme: a unit on Native Americans.
7. The number immediately to the left of the decimal point in the Arabic numeral system.
8. Mathematics.
   1. The lowest positive whole number; one.
   2. An element of a ring with a multiplicative inverse.

[Back-formation from UNITY.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.