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pro tempore (Latin) for a time; for the time being; temporarily. [2 definitions]
proteolysis the breaking down of proteins by gastric secretions into simpler, soluble compounds, as in digestion.
proteose any of several water-soluble compounds derived from the breakdown of proteins during digestion.
Proterozoic see Algonkian.
protest a statement or other expression of objection or dissent. [7 definitions]
Protestant any Christian, esp. in the West, who belongs to a non-Catholic church. [3 definitions]
Protestant Episcopal Church a U.S. church descended from and modeled on the Church of England but separate and autonomous since 1789; Episcopal Church.
Protestant ethic a set of beliefs in the moral virtue of productive work, self-discipline, and frugality; Protestant work ethic; work ethic.
protestation an emphatic or earnest declaration. [3 definitions]
protester one who formally declares oneself against a policy, action, or decision. [2 definitions]
Proteus in Greek mythology, a sea god who can change his shape or appearance at will.
prothallium a tiny, flat, circular mass, bearing the sexual organs on the underside, that is produced in the sexual generation of ferns, some mosses, and related plants.
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.