side chain

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An organic molecular structure in the form of branched open chain or ring.

Century Dictionary and Cyclopedia

1. n. 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.
2. n. 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.
3. n. Same as receptor. See immunity, 5.

Wiktionary

1. n. chemistry in organic chemistry, that part of a molecule attached to some core structure; a radical
2. n. 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).

GNU Webster's 1913

1. adj. (Chem.) a chain of atoms attached to the main structure of a large molecule, especially of a polymer.