δ is the lowercase (capital δ) of the fourth Greek letter Delta.
δ is usually used for:
1 δ bond in valence bond. ***
2. It means charged: δ-means negatively charged, and δ+means positively charged.
3. Names of two functions in mathematics
4. Stochastic model (Miller-Orr model)
5. In furniture drawing, it is used to represent the section thickness.