side chain

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An organic molecular structure in the form of a branched open chain or ring.

from The Century Dictionary.

• noun In locomotive engines, one of the chains fixed to the sides of the tender and engine for safety, should the central drag-bar give way.
• noun The driving-chain which transmits motion to the driving-wheels of a motor-car from the counter-shaft or compensating-shaft. One is placed outside of the body of the vehicle on each side, an arrangement which necessitates two chains, and is distinguished from the center-chain design, which uses one chain under the body and near the center of the vehicle.
• noun Same as receptor. See immunity, 5.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry in organic chemistry, that part of a molecule attached to some core structure; a radical
• noun biochemistry the variable parts of amino acids that extend from the peptide backbone in proteins; they are referred to as R-groups, which branch off the backbone at the alpha carbon (Cα).