What does "delta G" refer to in chemistry?

In chemistry, delta G refers to the change in Gibbs Free Energy of a reaction. Gibbs Free Energy refers to the energy in a chemical reaction that can be used to do work.

The Gibbs Free Energy of a reaction, delta G, can be calculated through the equation delta G = delta H - T*delta S. In the previous equation: delta H is the change in enthalpy, T is the temperature in degrees Kelvin and delta S is the change in entropy. If delta G is negative, a reaction is said to be spontaneous; if delta G is positive, the reaction is non-spontaneous.