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P (iou)



P, p. [pi:]
The sixteenth letter of the modern English alphabet and the fifteenth of the ancient Roman one, corresp. to Greek pi, Phoenician and Semitic pe. The simple letter has always represented a voiceless bilabial stop, but it is usually silent in the combinations pn-, ps-, pt- (repr. Greek), and sometimes also medially between m and another consonant, esp. in words where it is not etymological (Hampstead, Simpson). The digraph ph- is used (after Latin usage) to transliterate the Greek letter phi, the phonet. value of which is now identical with that of F; ph is thus in effect a separate consonant: see PH. Pl. P's, Ps.
I.
The letter and its sound.
mind one's P's and Q's be careful or particular as to one's words or behaviour. P-CELTIC.
The shape of the letter.
P trap a trap consisting of a U-bend the upper part of whose outlet arm is bent horizontal or nearly so.
II. Symbolical uses.
Used to indicate serial order; applied e.g. to the sixteenth (or often the fifteenth, either I or J being omitted) group or section, sheet of a book, etc.
[Initial letter of primary.] P wave, an earthquake wave of alternate compression and rarefaction (the faster of the two main kinds of wave transmitted through the earth: cf. S wave s.v. S, S 4).
Chemistry. [After pH.] (Lower-case p.) Used to denote the negative of the common logarithm of a concentration or activity expressed in moles per litre (e.g. pOH, that of hydroxyl ion concentration). Also pK, the negative of the common logarithm of a dissociation constant.
Astronomy. [Initial letter of proton.] p-process, a process believed to occur in stars by which heavy proton-rich nuclei are formed from other nuclei, esp. in circumstances of high proton flux (e.g. in supernovae).
III.
Abbrevs.: P = parking (on road signs); (Chess) pawn; (Physics) (as prefix) peta-; (Chemistry) phosphorus; (Linguistics) phrase (as in P-marker); (Chemistry) poly- (in PCB etc. below); (Physics) poise; pressure; (Telephony) private (in PABX etc. below); proprietary. p = page; (Chemistry) (as prefix) para-; (Biology) parental generation; penny, pence (in decimal currency); (Music) piano, softly; (as prefix) pico-; pressure; (Physics & Chemistry) principal: orig. designating one of the four main series (S, P, D, F) of lines in atomic spectra, now more frequently applied to electronic orbitals, states, etc., possessing one unit of angular momentum (as p-electron, p-orbital, etc.).