apostrophe

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The superscript sign ( ' ) used to indicate the omission of a letter or letters from a word, the possessive case, or the plurals of numbers, letters, and abbreviations.
2. n. The direct address of an absent or imaginary person or of a personified abstraction, especially as a digression in the course of a speech or composition.

Century Dictionary and Cyclopedia

1. n. In rhetoric, a digressive address; the interruption of the course of a speech or writing, in order to address briefly a person or persons (present or absent, real or imaginary) individually or separately; hence, any abrupt interjectional speech. Originally the term was applied only to such an address made to one present.
2. n. In botany, the arrangement of chlorophyl-granules under the action of direct sunlight (light-apostrophe), and in darkness (dark-apostrophe): in the first case upon the lateral walls of the cells, so that their edges are presented to the light; in the latter, upon the lateral and basal cell-walls: used in distinction from epistrophe (which see).
3. n. In grammar, the omission of one or more letters in a word.
4. n. In writing and printing, the sign (') used to indicate such omission. The omission may be of a letter or letters regularly written but not sounded, as in tho' for though, liv'd for lived, aim'd for aimed, etc.; of a letter or letters regularly sounded and written, and omitted only in poetical or colloquial speech, as in o'er for over, don't for do not, etc.; or of a letter regularly sounded but not written, as in the possessives church's, fox's, Jones's, etc., and so formerly often in similar plurals now written in full, as churches, foxes, Joneses. The apostrophe is now extended to all possessives (except of pronouns) as a mere sign of the case, as boy's, lion's, etc., also when the suffix is omitted, as in conscience' sake, and in plural possessives, as boys', lions'; and it is still used in some unusual or peculiar plurals, as many D. D.'s and LL.D.'s, a succession of a's, four 9's, etc.
5. n. The sign (') used for other purposes, especially, single or double, as a concluding mark of quotation, as in “‘Well done,' said he.” See quotation-mark.

Wiktionary

1. n. The text character ’, that serves as a punctuation mark in various languages and as a diacrictical mark in certain rare contexts.
2. n. A sudden exclamatory piece of dialogue addressed to someone or something, especially absent.

GNU Webster's 1913

1. n. A figure of speech by which the orator or writer suddenly breaks off from the previous method of his discourse, and addresses, in the second person, some person or thing, absent or present.”
2. n. The contraction of a word by the omission of a letter or letters, which omission is marked by the character ['] placed where the letter or letters would have been.
3. n. The mark ['] used to denote that a word is contracted (as in ne'er for never, can't for can not), and as a sign of the possessive, singular and plural; as, a boy's hat, boys' hats. In the latter use it originally marked the omission of the letter e.

WordNet 3.0

1. n. address to an absent or imaginary person
2. n. the mark (') used to indicate the omission of one or more letters from a printed word