d

from The American Heritage® Dictionary of the English Language, 5th Edition.

• abbreviation deuteron
• abbreviation diameter
• abbreviation differential
• abbreviation down quark
• noun The fourth letter of the modern English alphabet.
• noun Any of the speech sounds represented by the letter d.
• noun The fourth in a series.
• noun Something shaped like the letter D.
• noun The lowest passing grade given to a student in a school or college.
• noun The second tone in the scale of C major or the fourth tone in the relative minor scale.
• noun A key or scale in which D is the tonic.
• noun A written or printed note representing this tone.
• noun A string, key, or pipe tuned to the pitch of this tone.

from The Century Dictionary.

• A form of -ed, -ed, in certain words. See -ed, -ed.
• An abbreviation of the Latin defensor fidei, defender of the faith. See defender.
• noun An abbreviation of Latin (ML.) divinitatis doctor, Doctor of Divinity.
• An abbreviation of the Latin Dei gratia, by the grace of God.
• noun In music, an abbreviation of destra mano (which see).
• noun Abbreviations of Doctor of Music.
• An abbreviation of dead-reckoning.
• noun In music, an abbreviation of da capo.
• noun An abbreviation of dal segno.
• noun An abbreviation of Doomsday Book.
• The fourth letter and third consonant in the English alphabet: the corresponding character has the same position and the same value also in the Latin, Greek, and Phenician alphabets, from which it comes to us. (See A.)
• As a numeral, in the Roman system, D stands for 500; when a dash or stroke is placed over it, as D, it stands for 5,000.
• As a symbol: In music: The second tone, or re, of the scale of C.
• A note which represents this tone.
• The key-note of the key of two sharps .
• On the keyboard of the organ or pianoforte, the white key or digital included in each group of two black keys.
• The string in a stringed instrument that is tuned to the tone D, as the third string of the violin, etc. In chem., D is the symbol of didymium.
• In mathematics, d is the sign of differentiation, ∂ of partial differentiation, δ of variation, D of derivation (commonly in the sense of taking the differential coefficient), ▵ of differencing, and ⾿ of the Hamiltonian operator.
• In the mnemonic words of logic, the sign of reduction to darii.
• As an abbreviation: In Eng. reckoning (d. or d.), an abbreviation of denarius, the original name for the English penny: as, £ s. d., pounds, shillings, and pence; 2s. 1d., two shillings and one penny.
• Before a date (d.), an abbreviation of died.
• In dental formulas, an abbreviation of deciduous, prefixed without a period to the letters i, c, and m: thus, di., deciduous incisor; dc., deciduous canine; dm., deciduous molar: all being teeth of the milk-dentition of a diphyodont mammal.
• or, more simply, taking one half of each jaw only, di. , dc. , dm. . In either case the numbers above the line are those of the upper teeth, and those below the line of the under teeth. See dental.
• In anatomy and ichthyology (d. or D.), an abbreviation of dorsal (vertebra or fin, respectively).
• In a ship's logbook (d.), an abbreviation of drizzling.
• In music: Also, the key-note of the minor key medieval music, the final of the Dorian and Hypodorian modes.
• In chem.: d- before certain compounds has reference to their behavior toward polarized light, namely, to their dextrorotation, as distinguished from their inaction (i-) or levorotation (l-).
• In mathematics: D is also used for the number denoting the deficiency of a curve (what its number of double points lacks of the maximum).
• As an abbreviation: In law (D.), an abbreviation of Decree, Decret, Dictum.
• In medicine (d.), an abbreviation of: diopter or dioptric;
• dexter (right.);
• divide (in prescriptions).
• (D.) Of Democrat, Deus (God), Dominus (Lord), Dutch; (d.) of daughter, delete (cancel), density.
• Short for damn (often printed d—).
• Any mechanical device or appliance which resembles the letter D; specifically, in a harness, a loop of metal which has a straight bar joined at each end to a semicircular loop: used as a support for a strap. Also written dee. See D-trap and D-valve.
• noun An abbreviation of Dynamical Engineer, a degree conferred at the completion of a graduate course in mechanical engineering.