i

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The ninth letter of the modern English alphabet.
2. n. Any of the speech sounds represented by the letter i.
3. n. The ninth in a series.
4. n. Something shaped like the letter I.
5. The symbol for imaginary unit.

Century Dictionary and Cyclopedia

1. The ninth letter and third vowel in the English alphabet. The character comes, like most of its predecessors (see A, etc.), through the Latin and Greek from the Phenician, and ultimately perhaps from the Egyptian. The correspondences are as follows:
2. The Phenician character represented rather a consonant, a y, than a vowel, but it was converted to vowel value by the Greeks, and has continued to bear that value since (though in Latin used as consonant also). Our “short i” of it, etc., is not far from the original sound; yet nearer is the sound which we perversely call “long e” (of mete, meet, meat, etc.), or the i of machine, pique, etc. Because the words which anciently showed this latter sound have in great measure changed it to a diphthongal utterance (nearly ä + i, or the ai of aisle), we have come to call the altered sound “long i.” The true i-sounds (in pick, pique) are close vowels, made with as near an approximation of the organs as is possible without giving rise to a fricative utterance. The approximation is made by the upper flat surface of the tongue to the palate, at or near the point where a complete closure makes a k-sound. Hence the i-sound has palatal affinities, and it (as also in less degree the e) is widely active in palatalizing a consonant: for example, in converting in modern English a t to ch, a d to j, an s to sh, a z to zh; having in older English, and in other languages, a like influence on a k or g. Hence, also, it is a vowel close to a consonant, and very nearly identical with the consonantal y, into which it passes freely. (See Y.) I has also gained in many words before r the same sound that e and u have in the same situation: for example, fir, first. It enters into various digraphs, as ai, ei, ie, oi, ui.
3. As a symbol: The number one in the Roman notation. It is repeated for subsequent numbers up to three (formerly to four) (II, III, IIII). These numerals placed after symbols of higher numbers increase their value: as, VI, six; VII, seven, etc.; XII, twelve; LIII, fifty-three; formerly CIIII, one hundred and four. Instead of the old IIII and VIIII for four and nine, an I is now prefixed to V or X to decrease the value by one: thus, IV, four; IX, nine.
4. In logic, a symbol of the particular affirmative proposition: derived from the second vowel of the Latin word affirmo, I assert. See A, 2 .
5. In chem., the symbol for iodine.
6. An abbreviation
7. In dental formulæ, in zoology, for incisor.
8. Same as i. e.
9. See i. e., i. q.
10. The nominative case of the pronoun of the first person; the word by which a speaker or writer denotes himself.
11. n. The pronoun I used as a substantive.
12. n. In metaphysics, the object of self-consciousness; that which is conscious of itself as thinking, feeling, and willing; the ego.
13. An obsolete form of aye.
14. n. An occasional obsolete spelling of eye.
15. n. A light form of in: as, “a worm i' the bud,’
16. n. A prefix (often spelled y-, and sometimes e- and a-) common in Middle English, as in i-blent, i-cast, i-don, i-take, i-cleped, i-wis, etc. (also spelled y-blent, y-cast, y-don, etc.), but entirely lost in modern English, except as traces remain in y-wis, adv. (sometimes erroneously written I wis), and in y-clept and a few other archaic perfect-participle forms affected by Spenser and other poets, and in alike, along, among, enough, everywhere, handiwork, and a few other common words in which the syllable concerned is not now recognized as a prefix. This prefix was extremely common in Anglo-Saxon, being used with nouns, adjectives, pronouns, and adverbs (having with these a collective or generalizing force, often so indefinite as not to be felt), but especially with verbs (having with these a collective force often translatable by together or with, or a completive or transitive force, and hence much used in the formation of transitive from intransitive verbs); in many instances it added nothing to the force of the verb to which it was prefixed. In Anglo-Saxon many verbs, as in German all verbs, without this or another prefix in the finite forms, take it in the past participle.
17. n. A form of the negative prefix in- before gn- in some words of Latin origin, as in ignoble, ignore, ignorant, etc.
18. n. An apparent connective, but properly a prefix, in hand-i-work and hand-i-craft (altered from hand-craft in imitation of handiwork), and (now spelled -y-) in ever-y-where. See these words, and compare i-.
19. n. The usual ‘connecting vowel,’ properly the stem-vowel of the first element, of compound words taken or formed from the Latin, as in mult-i-form, cent-i-ped, ens-i-form, omn-i-potent, aur-i-ferous, bell-i-gerent, etc. In forming New Latin compounds, the vowel is regularly -i-, as scut-i-fera [⟨ L. scutum (scuto-) + -fera], even when the second element is Greek, as scut-i-phora [⟨ L. scutum (scuto-), + Gr. φόρος]; but in the latter case the vowel -o-, proper to Greek compounds, is often used, as scut-o-pterus [⟨ L. scutum (scuto-) + Gr. πτερόν]. Even when both elements are Latin, the connective -o- is sometimes used; but it is properly confined to Greek and other non-Latin compounds. See -o-.
20. n. In philology an abbreviation of Indo-Euro-pean.
21. The usual symbol for the moment of inertia.
22. In electricity, a symbol for current.
23. In mathematics: The symbol (i or i) for the neomon, the square root of minus one (√—1, (—1)). In quaternions, the symbols i, j, k denote a system of three right versors in three mutually rectangular planes; thus i is a particular quaternion having for its amplitude one right angle.
24. In chem., i- before certain compounds has reference to their inaction as distinguished from dextro-rotation (d-) or levorotation (1-).
25. An abbreviation of Idaho;
26. of the Latin Imperator, emperor;
27. of Island;
28. of intransitive.
29. A nominative plural ending of Latin masculine nouns and adjectives of the ‘second’ declension, with nominative singular in -us, or without suffix, many of which have come into English use, literary or technical. Examples are acini, cyathi, denarii, foci, genii, hippopotami, illuminati, literati, loci, ocelli, radii, Galli, Iberi, Chatti, etc. In some instances there is also a regular English plural in -es after the ending -us, as focuses, geniuses, hippopotamuses, etc. This plural suffix appears in many classnames in zoology and botany which are plurals of individual or generic names in -us which are less often used in the singular. Examples are Acanthopterygii, Chondropterygii (sc. pisces, fish), Acrocarpi (sc. musci, mosses), etc. See also -ini, etc.
30. A nominative plural suffix of Italian nouns sometimes used in English, as banditti, dilettanti, lazzaroni, scudi, soprani, etc.
31. The ending of some Latin genitives singular of nouns and adjectives of the second declension, occurring in some ancient, medieval or modern Latin phrases used in English, as genius loci, lapis lazuli, quid novi, etc.

Wiktionary

1. The ninth letter of the alphabet, called [[i#|i]] and written in the Latin script.
2. The ordinal number ninth, derived from this letter of the alphabet, called [[i#|i]] and written in the Latin script.
3. n. The name of the Latin script letter [[I#|I]]/[[i#|i]].
4. pro. Obsolete capitalization of Xyzy}}.

GNU Webster's 1913

1. I, the ninth letter of the English alphabet, takes its form from the Phœnician, through the Latin and the Greek. The Phœnician letter was probably of Egyptian origin. Its original value was nearly the same as that of the Italian I, or long e as in mete. Etymologically I is most closely related to e, y, j, g; as in dint, dent, beverage, L. bibere; E. kin, AS. cynn; E. thin, AS. þynne; E. dominion, donjon, dungeon.
2. In our old authors, I was often used for ay (or aye), yes, which is pronounced nearly like it.
3. As a numeral, I stands for 1, II for 2, etc.
4. pro. The nominative case of the pronoun of the first person; the word with which a speaker or writer denotes himself.

WordNet 3.0

1. n. the 9th letter of the Roman alphabet
2. n. a nonmetallic element belonging to the halogens; used especially in medicine and photography and in dyes; occurs naturally only in combination in small quantities (as in sea water or rocks)
3. n. the smallest whole number or a numeral representing this number
4. adj. used of a single unit or thing; not two or more