Water activity has been universally used in the pharmaceutical industry since the publication in 2006 of USP. It is defined as the energy status of water in a system and is rooted in the fundamental laws of thermodynamics through the Gibbs free energy equation. It represents the relative chemical potential energy of water as dictated by the surface, colligative, and capillary interactions in a matrix. In the next lines, Dr. Brady Carter at Carter Scientific Solutions discusses this topic further.