- A chemical made up of two or more atoms. The atoms in a molecule can be the same (an oxygen molecule has two oxygen atoms) or different (a water molecule has two hydrogen atoms and one oxygen atom). Biological molecules such as proteins and DNA can be made up of many thousands of atoms.
Definition from: Physician Data Query via Unified Medical Language System at the National Library of Medicine
- The smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
Related discussion in the Handbook
See also Understanding Medical Terminology.