| Phototroph | (Environmental Engineering) Organisms which obtain energy from light using photooxidation. |
| POTW | (Environmental Engineering) or Publicly Owned Treatment Works Any municipally owned wastewater treatment facility. |
| Primary standards | (Environmental Engineering) Required drinking water quality standards related directly to human health. These standards are required and enforceable by the U.S. EPA. See secondary standards. |
| Primary treatment | (Environmental Engineering) Treatment which includes all operation prior to and including primary treatment, e.g., bar screening, grit removal, comminution, and primary sedimentation. |
| Procaryotic organisms | (Environmental Engineering) Organisms which do not have a cellular membrane. |
| Producers | (Environmental Engineering) Autotrophic organisms which produce protoplasm using inorganic carbon and energy from the sun. |
| Publicly owned treatment works (POTW) | (Environmental Engineering) A municipal or domestic wastewater treatment facility. |
| Pair programming - | (Software Engineering) two people work together (side-by-side) to design and construct a software component, providing real-time problem solving and quality control. |
| Paper prototype - | (Software Engineering) a paper representation of an application (e.g., story boards that describe the interaction at a human interface) |
| Paradigms - | (Software Engineering) the process model |