- Any nonprotein substance required by a protein for biological activity, such as prosthetic groups of coenzymes, which are not consumed in the process and are found unchanged at the end of the reaction.
Definition from: CRISP Thesaurus via Unified Medical Language System at the National Library of Medicine
- 1) A substance that acts with another substance to bring about certain effects;
2) Something (as a diet or virus) that acts with or aids another factor in causing disease.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.