Permeability: The ability of a rock formation to transmit fluids. A measure of how much resistance a rock formation has to the movement of fluids through it. Formations where fluids readily move through them are permeable and typically have many large, well-connected pores. Low-permeability formations typically are made up of finer grains or mixed grains with smaller, fewer, or less-interconnected pores. Permeability is measured in darcys.