Etymology
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transparency (n.)
1610s, "condition of being transparent," from Medieval Latin transparentia, from transparentem (see transparent). Meaning "that which is transparent" is from 1590s; of pictures, prints, etc., from 1785; in photography from 1866. Related: Transparence.
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perspicuity (n.)

late 15c., perspicuite, of things, "clearness, transparency" (a sense now obsolete); 1540s of words or expressions, "quality of being clear to the mind; quality by which the meaning can be seen through the words," from Old French perspicuité and directly from Latin perspicuitas "transparency, clearness," from perspicuus "transparent," from perspicere "look through, look closely at" (see perspective).

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lucidity (n.)
1650s, "brightness," from French lucidité, from Late Latin luciditas, from Latin lucidus "light, bright, clear," from lucere "to shine," from PIE *louk-eyo-, suffixed (iterative) form of root *leuk- "light, brightness." Meaning "intellectual clarity, transparency of expression" is by 1851.
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clear (v.)

mid-14c., "make clear (an obscure subject) in the mind, explain, elucidate;" late 14c., "make clean, cleanse, purify; clarify (a liquid), remove what clouds or diminishes brightness or transparency;" also "prove innocent, vindicate;" of the weather, sea, sky, clouds, etc., "clear up, become fair or calm;" from clear (adj.).  Related: Cleared; clearing.

Intransitive sense of "become free from murkiness" is from 1580s. Meaning "to free from obstructions" is from 1520s; that of "to free from entanglement" is from 1590s; that of "pass (an obstacle) without entanglement or collision" is from 1630s. Sense of "to remove (something) out of the way" is from 1670s; that of "to clear land of trees and underbrush" is from 1690s. Meaning "to leap clear over" is first attested 1791. Meaning "to gain (a sum of money) in clear profit" is from 1719. Meaning "get approval for (a proposal, etc.) from authority" is from 1944; meaning "establish as suitable for national security work" is from 1948.

To clear (one's) throat is from 1881; earlier clear (one's) voice (1701). To clear out "depart, leave" (1825), perhaps is from the notion of ships satisfying customs, harbor regulations, etc., then setting sail. To get clear of is from 1590s. To clear up is from 1620s of weather, 1690s as "make clear to the mind." Clear the deck (1802) is from sailing ships. Clear the air in the figurative sense is from late 14c. To clear the coast (1520s) was to make it suitable for landing.

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porcelain (n.)

ceramic ware having a translucent body, and, when it is glazed, a translucent glaze, 1530s, from French porcelaine and directly from Italian porcellana "porcelain" (13c.), literally "cowrie shell;" the chinaware being so called from resemblance of its lustrous transparency to the shiny surface of the shells. As an adjective from 1590s.

The shell's name in Italian is from porcella "young sow," fem. of Latin porcellus "young pig," diminutive of porculus "piglet," itself a diminutive of porcus "pig" (from PIE root *porko-"young pig"). Compare Greek khorinē "cowrie," also literally "a little pig." 

According to an old theory, the connection of the shell and the pig is a perceived resemblance of the shell (also Venus shell) opening to the exposed outer genitalia of pigs. For a different answer, Century Dictionary (1897) writes that the shell was "so called because the shape of the upper surface resembles the curve of a pig's back."

Thompson ["A Glossary of Greek Fishes"] writes, "According to a widespread belief the cowries were a charm or talisman against sterility. They are among the women's ornaments at Pompeii, and are found in women's graves in France and England as late as the Middle Age ...." He writes that the bigger, showier shells, from the Red Sea, probably account for the "popular and erotic names," but adds that the association of the shells with female sex is almost worldwide.

porcelain is china & china is p.; there is no recondite difference between the two things, which indeed are not two, but one; & the difference between the two words is merely that china is the homely term, while porcelain is exotic & literary. [Fowler]
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onion (n.)

early 12c., ungeon, oinyon, unione, "the underground bulb of the common onion plant," from Anglo-French union, Old French oignon "onion" (formerly also oingnon), and directly from Latin unionem (nominative unio), a colloquial rustic Roman word for a kind of onion, also "pearl" (via the notion of a string of onions), literally "one, unity." The sense connection is the unity of the successive layers of an onion, in contrast with garlic or cloves.

Old English had ynne (in ynne-leac), from the same Latin source, which also produced Irish inniun, Welsh wynwyn and similar words in Germanic. In Dutch, the ending in -n was mistaken for a plural inflection and new singular ui formed. The usual Indo-European name is represented by Greek kromion, Irish crem, Welsh craf, Old English hramsa, Lithuanian kermušė.

The usual Latin word was cepa, a loan from an unknown language; it is the source of Old French cive, Old English cipe, and, via Late Latin diminutive cepulla, Italian cipolla, Spanish cebolla, Polish cebula. German Zwiebel also is from this source, but altered by folk etymology in Old High German (zwibolla) from words for "two" and "ball."

Onion-ring "circular segment of an onion" (especially battered and deep-fried) is attested by 1904. Onion-dome on a church-tower, etc., is attested by 1950, so called for the resemblance of shape; onion-grass, which forms tuberous nodes in its roots (also onion-couch) is from 1823; onion-skin as a type of paper (so called for its thinness, transparency, and finish, which resemble the skin of an onion) is from 1879.

Onions, the surname, is attested from mid-12c. (Ennian), from Old Welsh Enniaun, ultimately from Latin Annianus, which was associated with Welsh einion "anvil."

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