Secondary Structure
Local regular folding patterns of a polypeptide backbone stabilised by hydrogen bonds.
Unit
Dimensionless
In depth
The alpha helix and beta pleated sheet are the two principal motifs, both stabilised by hydrogen bonds between backbone carbonyl and amide groups rather than between side chains. Which forms where depends on the local amino acid sequence.
Examples in the real world
The alpha helix of keratin in hair; the beta sheets of silk fibroin giving it tensile strength.