1. the fourth letter of the Greek alphabet, Δ or δ; used in chemical names to denote the fourth of a series of isomeric compounds or the carbon atom fourth from the carboxyl group, or to denote the fourth of any series.
2. a triangular area.
- d. T lymphocyte — see T lymphocyte.

, the average change of y per unit x, i.e. the change of y over the change of x

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