Δ, δ

1. Fourth letter of the Greek alphabet, delta. 2. In chemistry, denotes a double bond, usually with a superscript to indicate position in a chain (Δ⁵); application of heat in a reaction (A B); absence of heat treatment (/); distance between two atoms in a molecule; or position of a substituent located on the fourth atom from the carboxyl or other primary functional group (δ); change (Δ); thickness (δ); chemical shift in NMR (δ).