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proton an elementary particle of matter in the nucleus of all atoms, having a positive electrical charge.
proton synchrotron a ring-shaped synchrotron that is used to accelerate protons and other heavy particles to energies of billions of electron-volts.
protoplanet a large body of matter that is a precursor to a planet.
protoplasm a complex translucent colorless jelly regarded as the physical basis of all living matter and life functions.
protostar a collapsing cloud of gas and dust that will become a star once nuclear fusion has begun.
prototype an original model on which later stages or forms are based or developed. [3 definitions]
protozoan any of the simplest forms of animal life, consisting of a single cell or a colony of single cells.
protract to extend in time; prolong. [3 definitions]
protractile capable of being protracted, extended, or protruded.
protractor a semicircular instrument with graduated markings, used in mathematics, surveying, and the like to construct and measure angles. [2 definitions]
protrude to push or extend outward; project. [2 definitions]
protrusile capable of being protruded or thrust out, as a hummingbird's tongue or an elephant's trunk.
protrusion the act of protruding or the state of being protruded. [2 definitions]
protrusive protruding or tending to protrude.
protuberance that which projects; bulge or bump. [2 definitions]
protuberant bulging outward.
proud feeling pleased or satisfied with oneself because of an accomplishment, attribute, or possession. [6 definitions]
proud flesh see granulation tissue.
prove to show (something) to be true, valid, or genuine. [4 definitions]
proven a past participle of prove.
provenance the source of something; place of origin.