Porosity: The percentage of pore space, or pore volume (in other words, the volume within sedimentary rock that can contain fluids). Primary porosity, such as spaces between grains that were not compacted or cemented together completely, develops during sedimentary rock deposition. Secondary porosity is developed when rock is altered, such as when some mineral grains dissolve from the rock. Fracture porosity can be generated by the development of fractures. Effective porosity is the interconnected pore volume in a rock that supports fluid flow in a reservoir. Total porosity is the total void space in the rock including isolated pores. Typically, effective porosity is less than total porosity.