1. Biologically, any rhythmic center that establishes a pace of activity.
2. An artificial regulator of rate activity.
3. In chemistry, the substance of which the rate of reaction sets the pace for a series of chain reactions; the rate-limiting reaction itself, in a metabolic pathway, the enzyme catalyzing the slowest or rate-limiting reaction in that pathway.
Stedman’s Medical Dictionary © Wolters Kluwer Health. All rights reserved.