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] |