Etymology
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blossom (n.)
c. 1200, from Old English blostm, blostma "a flower of a plant," from Proto-Germanic *blo-s- (source also of Middle Low German blosom, Dutch bloesem, German Blust), from PIE *bhlow-, extended form of root *bhel- (3) "to thrive, bloom." This is the native word, now largely superseded by bloom (n.1) from Old Norse and flower (n.) from French.
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blossom (v.)
late 14c., from Old English blostmian "put forth blossoms, to flower," from blostma "a blossom, a flower" (see blossom (n.)). Figurative use from late 14c. Related: Blossomed; blossoming.
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orange-blossom (n.)

"fragrant, white blossom of the orange tree," 1786, from orange (n.) + blossom (n.). Especially as worn by brides; "the custom appears to have been introduced from France c 1820-30" [OED].

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bloom (n.1)

"blossom of a plant," c. 1200, a northern word, from a Scandinavian source akin to Old Norse blomi "flower, blossom," also collectively "flowers and foliage on trees;" from Proto-Germanic *blomon (source also of Old Saxon blomo, Middle Dutch bloeme, Dutch bloem, Old High German bluomo, German Blume, Gothic bloma), from PIE *bhle- (source also of Old Irish blath "blossom, flower," Latin flos "flower," florere "to blossom, flourish"), extended form of root *bhel- (3) "to thrive, bloom." Related to Old English blowan "to flower" (see blow (v.2)).

Not extended like 'flower' to a whole 'flowering plant', and expressing a more delicate notion than 'blossom', which is more commonly florescence bearing promise of fruit, while 'bloom' is florescence thought of as the culminating beauty of the plant. Cherry trees are said to be in blossom, hyacinths in bloom. [OED]

Transferred sense, of persons, "pre-eminence, superiority," is from c. 1300; meaning "state of greatest loveliness" is from early 14c.; that of "blush on the cheeks" is from 1752. Old English had cognate bloma, but only in the figurative sense of "state of greatest beauty;" the main word in Old English for "flower" was blostm (see blossom (n.)).

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flower (n.)

c. 1200, flour, also flur, flor, floer, floyer, flowre, "the blossom of a plant; a flowering plant," from Old French flor "flower, blossom; heyday, prime; fine flour; elite; innocence, virginity" (12c., Modern French fleur), from Latin florem (nominative flos) "flower" (source of Italian fiore, Spanish flor), from PIE root *bhel- (3) "to thrive, bloom."

From late 14c. in English as "blossoming time," also, figuratively, "prime of life, height of one's glory or prosperity, state of anything that may be likened to the flowering state of a plant." As "the best, the most excellent; the best of its class or kind; embodiment of an ideal," early 13c. (of persons, mid-13c. of things); for example flour of milk "cream" (early 14c.); especially "wheat meal after bran and other coarse elements have been removed, the best part of wheat" (mid-13c.). Modern spelling and full differentiation from flour (n.) is from late 14c.

In the "blossom of a plant" sense it ousted its Old English cognate blostm (see blossom (n.)). Also used from Middle English as a symbol of transitoriness (early 14c.); "a beautiful woman" (c. 1300); "virginity" (early 14c.). Flower-box is from 1818. Flower-arrangement is from 1873. Flower child "gentle hippie" is from 1967.

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abloom (adj.)

"in blossom, in a blooming state," 1855, from a- (1) + bloom (v.).

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flourish (v.)
c. 1300, "to blossom, grow" (intransitive), from Old French floriss-, stem of florir "to blossom, flower, bloom; prosper, flourish," from Latin florere "to bloom, blossom, flower," figuratively "to flourish, be prosperous," from flos "a flower" (from PIE root *bhel- (3) "to thrive, bloom"). Metaphoric sense of "thrive" is mid-14c. in English. Transitive meaning "brandish (a weapon), hold in the hand and wave about" is from late 14c. Related: Flourished; flourishing.
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effloresce (v.)
"to come into flower," 1775, from Latin efflorescere, inceptive form (in Late Latin simplified to efflorere) "to blossom, spring up, flourish, abound," from assimilated form of ex "out" (see ex-) + florescere "to blossom," from flos "flower" (from PIE root *bhel- (3) "to thrive, bloom"). Sense in chemistry is from 1788.
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efflorescence (n.)

1620s, "a bursting into flower, act of blossoming out," from French efflorescence, from Latin efflorescentem (nominative efflorescens), present participle of efflorescere "to bloom, flourish, blossom," from assimilated form of ex "out" (see ex-) + florescere "to blossom," from flos "flower" (from PIE root *bhel- (3) "to thrive, bloom"). Sense in chemistry is from 1660s.

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florescence (n.)
"process of flowering," 1764, from Modern Latin florescentia, from Latin florescentem (nominative florescens) "blooming," present participle of florescere "to begin to bloom," inceptive of florere "to blossom" (see flourish (v.)).
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