In organic chemistry, a moiety ( ) is a part of a molecule that is given a name because it is identified as a part of other molecules as well. 
Typically, the term is used to describe the larger and characteristic parts of organic molecules, and it should not be used to describe or name smaller functional groups  of atoms that chemically react in similar ways in most molecules that contain them. Occasionally, a moiety may contain smaller moieties and functional groups.
A moiety that acts as a branch extending from the backbone of a hydrocarbon molecule is called a substituent or side chain, which typically can be removed from the molecule and substituted with others.
The term is also used in pharmacology, where an active moiety is the part of a molecule responsible for the physiological or pharmacological action of a drug.