| 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] |