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prothrombin a protein in blood plasma that combines with calcium during clotting to form thrombin.
protium the nonradioactive, most common isotope of hydrogen, having one proton and no neutrons in the nucleus of each atom. (Cf. deuterium, tritium.)
proto- first; earliest; primitive.
protocol the rules pertaining to etiquette and ceremonial observances among diplomats, heads of state, and the military. [5 definitions]
protohuman an early primate that resembled modern man.
Proto-Indo-European see Indo-European.
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]