-a (1)
word-forming element which in English is characteristic of fem. nouns and adjectives of Latin or Greek origin (such as idea, coma, mania, basilica, arena, formula, nebula). From Latin -a (plural -ae) and Greek -a, (plural -ai, Latinized as -ae). The Latin suffix also became Italian -a (plural -e), Spanish -a (plural -as). It is represented in Old English by -u, -e, but even then the suffix was fading and by the time of modern English was totally lost or swallowed into silent final -e-.

It also appears in Romanic words from Latin that have been borrowed into English, such as opera, plaza, armada. It figures in scientific names coined in Modern Latin (amoeba, soda, magnolia, etc.) and is common in geographical names formed according to Latin or Greek models (Asia, Africa, America, Arabia, Florida, etc.)

In English it marks sex only in personal names (Julia, Maria, Alberta) and in a few words from Italian or Spanish where a corresponding male form also is in use (donna, senora).
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-a (2)
nominative neuter plural ending of certain nouns and adjectives in Latin and Greek that have been adopted into English (phenomena, data, media, criteria, etc.). It also is common in biology in Modern Latin formations of class names (Mammalia, Reptilia, Crustacea).
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a- (2)
word-forming element meaning "away," from Latin a "off, of, away from," the usual form of Latin ab before consonants (see ab-). As in avert, avocation. It is also the a in a priori and the à in Thomas à Kempis, Thomas à Becket.
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word-forming element expressing ability, fitness, or capacity, from Latin -abilitas, forming nouns from adjectives ending in -abilis (see -able). Not etymologically related to ability, though popularly connected with it.
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common termination and word-forming element of English adjectives (typically based on verbs) and generally adding a notion of "capable of; allowed; worthy of; requiring; to be ______ed," sometimes "full of, causing," from French -able and directly from Latin -abilis. It is properly -ble, from Latin -bilis (the vowel being generally from the stem ending of the verb being suffixed), and it represents PIE *-tro-, a suffix used to form nouns of instrument, cognate with the second syllables of English rudder and saddle (n.).

A living element in English, used in new formations from either Latin or native words (readable, bearable) and also with nouns (objectionable, peaceable). Sometimes with an active signification (suitable, capable), sometimes of neutral signification (durable, conformable). It has become very elastic in meaning, as in a reliable witness, a playable foul ball, perishable goods. A 17c. writer has cadaverable "mortal."

To take a single example in detail, no-one but a competent philologist can tell whether reasonable comes from the verb or the noun reason, nor whether its original sense was that can be reasoned out, or that can reason, or that can be reasoned with, or that has reason, or that listens to reason, or that is consistent with reason; the ordinary man knows only that it can now mean any of these, & justifiably bases on these & similar facts a generous view of the termination's capabilities; credible meaning for him worthy of credence, why should not reliable & dependable mean worthy of reliance & dependence? [Fowler]

In Latin, -abilis and -ibilis depended on the inflectional vowel of the verb. Hence the variant form -ible in Old French, Spanish, English. In English, -able tends to be used with native (and other non-Latin) words, -ible with words of obvious Latin origin (but there are exceptions). The Latin suffix is not etymologically connected with able, but it long has been popularly associated with it, and this probably has contributed to its vigor as a living suffix.

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word-forming element in Modern Latin making names for orders and classes in zoology (Crustacea, Cetacea, etc.), from Latin -acea, neuter plural of -aceus "belonging to, of the nature of" (enlarged from adjectival suffix -ax, genitive -acis).

The names are thus formally adjectives, Latin animalia "animals" (a neuter plural noun) being understood. Thus Crustacea "shellfish" are *crustacea animalia "crusty animals."

In botany, the suffix is -aceae, from the fem. plural of -aceus, forming orders or families of plants (Rosaceae, etc.) with a presumed plantae "plants," which is a fem. plural.
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word-forming element denoting "belonging to, of the nature of," from Latin -aceus, enlarged form of adjectival suffix -ax (genitive -acis); see -acea. Especially in biology, "pertaining to X order of plants or animals."
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compound adjectival word-forming element of Latin origin, attached to verb stems and expressing intensity of action: "given to, inclined to, abounding in," or expressing intensity of physical or mental action, from Latin -aci- (nominative -ax, accusative -acem), noun ending used with verbal stems (see -acea), + -ous. The accompanying nouns are formed in -acity.
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word-forming element making nouns of quality, state, or condition, a confusion in English of three similar suffixes from Latin:

1. in primacy, etc., from Old French -acie and directly from Medieval Latin -acia, Late Latin -atia, making nouns of quality, state, or condition from nouns in -as.

2. in advocacy, etc., from Late Latin -atia, forming nouns of state from nouns in -atus.

3. in fallacy, etc., from Latin -acia, forming nouns of quality from adjectives in -ax (genitive -acis). Also forming part of -cracy. It has been extended in English to nouns not found in Latin (accuracy) and to non-Latin words (piracy).

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word-forming element of Greek origin appended to nouns and denoting collective numerals (triad, Olympiad) and fem. patronymics (Dryad, Naiad, also, in plural, Pleiades, Hyades), thence also plant family names; from Greek -as (genitive -ados), fem. suffix equivalent to -is.

From its use in Iliad (literally "of Ilion," that is, "Troy;" from Ilias poiesis or oidos "poem of Ilion," the accompanying noun being feminine, hence the termination) it has formed titles of poems in imitation of it (Columbiad, Dunciad).

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