The Reynolds number (Re) helps predict flow patterns in different fluid flow situations.

The number of turns in one minute.

In thermodynamics, a reversible process is a process whose direction can be reversed to return the system to its original state by inducing infinitesimal changes to some property of the system's surroundings.

Reverse engineering is a process or method through the application of which one attempts to understand through deductive reasoning how a device, process, system, or piece of software accomplishes a task with very little (if any) insight into exactly how it does so.

Reverse costing describes the process of disassembling (reverse engineering) a device to identify manufacturing technology and calculate its manufacturing costs through a cost analysis of its parts and the effort required to assemble them.

A ratio that describes the profitability or efficiency of an investment.

The addition of new technology or features to older systems.

Pico hydro is a term used for hydroelectric power generation of under 5 kW.

A gas field that is too deep to drill for, or is beneath an obstruction, may be considered physically stranded.

A physical quantity is any phenomenon that can be measured with an instrument or be calculated for.

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry.

Photovoltaic thermal collectors, typically abbreviated as PVT collectors and also known as hybrid solar collectors, photovoltaic thermal solar collectors, PV/T collectors or solar cogeneration systems, are power generation technologies that convert solar radiation into usable thermal and electrical energy.

Photovoltaic (PV) technology transforms the energy of sunlight (solar photons) into direct electric current using semiconductor materials.

Photovoltaic system performance is the ratio of actual solar PV system output compared with expected values, which is essential for proper solar PV facility's operation and maintenance.

See "Solar Farm".

A photovoltaic system, also PV system or solar power system, is a power system designed to supply usable solar power by means of photovoltaics.

A physical and chemical phenomenom that generates voltage and current in materials that are exposed to light.

Also known as a solar cell, is used to convert light energy into electricity.

An interconnected system of PV modules that function as a single electricity-producing unit. In smaller systems, an array can consist of a single module.