Backbone chain

Main chain
Backbone
That linear chain to which all other chains, long or short or both,
may be regarded as being pendant.

Note: Where two or more chains could equally be considered to be the
main chain, that one is selected which leads to the simplest representation
of the molecule.

In polymer science, the backbone chain of a polymer is the longest series of covalently bonded atoms that together create the continuous chain of the molecule. This science is subdivided into the study of organic polymers, which consist of a carbon backbone, and inorganic polymers which have backbones containing only main group elements.

In biochemistry, organic backbone chains make up the primary structure of macromolecules. The backbones of these biological macromolecules consist of central chains of covalently bonded atoms. The characteristics and order of the monomer residues in the backbone make a map for the complex structure biological polymers. The backbone is, therefore, directly related to biological molecules’ function. The macromolecules within the body can be divided into four main subcategories, each of which are involved in very different and important biological processes: Proteins, Carbohydrates, Lipids, and Nucleic acids. Each of these molecules has a different backbone and consists of different monomers each with distinctive residues and functionalities. This is the driving factor of their different structures and functions in the body. Although lipids have a "backbone," they are not true biological polymers as their backbone is a three carbon molecule, glycerol, with longer substituent "side chains." For this reason, only proteins, carbohydrates, and lipids should be considered as biological macromolecules with polymeric backbones.

Polymer Chemistry:

The character of the backbone chain depends on the type of polymerization: in step-growth polymerization, the monomer moiety becomes the backbone, and thus the backbone is typically functional. These include polythiophenes or low band gap polymers in organic semiconductors. In chain-growth polymerization, typically applied for alkenes, the backbone is not functional, but bears the functional side chains or pendant groups.

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Wikipedia