Passive nuclear safety is a design approach for safety features, implemented in a nuclear reactor, that does not require any active intervention on the part of the operator or electrical/electronic feedback in order to bring the reactor to a safe shutdown state.

Passive house is a voluntary standard for energy efficiency in a building, which reduces the building's ecological footprint.

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption.

Passivation, in physical chemistry and engineering, refers to a material becoming "passive," that is, less affected or corroded by the environment of future use.

Pascal's law or the principle of transmission of fluid-pressure) is a principle in fluid mechanics given by Blaise Pascal that states that a pressure change at any point in a confined incompressible fluid is transmitted throughout the fluid such that the same change occurs everywhere.

The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength.

In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction.

Particulates are microscopic particles of solid or liquid matter suspended in the air.

Waste products from the combustion process that is emitted with flue gases, water and solids.

Particle physics is a branch of physics that studies the nature of the particles that constitute matter and radiation.

A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature.

The Paris Agreement is an agreement within the United Nations Framework Convention on Climate Change (UNFCCC), on climate change mitigation, adaptation, and finance, signed in 2016.

With regard to electricity production, parasitic loss it is any of the loads or devices powered by the generator, not contributing to net electric yield found by subtracting productive yield from gross yield.

In electrical networks, a parasitic element is a circuit element (resistance, inductance or capacitance) that is possessed by an electrical component but which it is not desirable for it to have for its intended purpose.

If two or more components are connected in parallel, they have the same difference of potential (voltage) across their ends.

Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal or oil shale that consists of a mixture of hydrocarbon molecules containing between twenty and forty carbon atoms.

A parabolic reflector that concentrates light onto a receiver filled with a working fluid flowing through the receiver. The resulting heat source is used for power generation.

With a parabolic dish collector, one or more parabolic dishes concentrate solar energy at a single focal point, similar to the way a reflecting telescope focuses starlight, or a dish antenna focuses radio waves.

A pantograph is an apparatus mounted on the roof of an electric train or tram to collect power through contact with an overhead line.