1590s, "force of expression," from French énergie (16c.), from Late Latin energia, from Greek energeia "activity, action, operation," from energos "active, working," from en "at" (see en- (2)) + -ergos "that works," from ergon "work, that which is wrought; business; action" (from PIE root *werg- "to do").

Used by Aristotle with a sense of "actuality, reality, existence" (opposed to "potential") but this was misunderstood in Late Latin and afterward as "force of expression," as the power which calls up realistic mental pictures. Broader meaning of "power" in English is first recorded 1660s. Scientific use is from 1807. Energy crisis first attested 1970.

mid-15c., "of, pertaining to, or determined by the sun," from Latin solaris "of the sun," from sol "sun" (from PIE root *sawel- "the sun"). Meaning "living room on an upper story" (also sollar) is from Old English, from Latin solarium (see solarium). For "of or proceeding from the sun," the earlier word was Old English sunlic, Middle English sonneli. Solific (1550s) also has been used.

The meaning "operated by means of the sun or its heat" is from 1740; solar power is attested from 1915, solar cell as a photovoltaic device from 1955, solar panel, designed to absorb the sun's rays, is from 1964. The astronomical solar system "sun and the bodies revolving round or dependent on it" is attested from c. 1704; solar wind is so called from 1958.

Solar plexus (1771) "complex of nerves in the pit of the stomach," apparently so called from its central position in the body (see plexus).

1680s, in anatomy, "an interlacing of nerves, vessels, or fibers," Modern Latin, literally "braid, network," noun use of past participle of Latin plectere "to twine, braid, fold," from suffixed form of PIE root *plek- "to plait." Original use in solar plexus "network of nerves in the abdomen" (see solar). General sense of "net-like arrangement of parts" is from 1760s. Related: Plexal.

"vigor, energy," 1912, shortened form of pepper (n.), which was used in the figurative sense of "spirit, energy" from at least 1847. Pep rally "meeting to inspire enthusiasm" is attested from 1915; pep talk is from 1926. To pep (something) up "fill or inspire with vigor or energy" is from 1925.

1927, in psychoanalysis jargon, "charged with mental energy, emotionally loaded," a back-formation from cathectic "invested with emotional energy" (1927), which is from Latinized form of Greek kathektikos, from kathexis "holding, retention" (see cathexis).

name of a Semitic solar deity worshiped, especially by the Phoenicians and Carthaginians, "with much license and sensuality" [Century Dictionary], late 14c., Biblical, from Late Latin Baal, Greek Baal, from Hebrew Ba'al, literally "owner, master, lord," a title applied to any deity (including Jehovah; see Hosea ii.16), but later a name of the particular Phoenician solar deity; from ba'al "he took possession of," also "he married;" related to or derived from the Akkadian god-name Belu (source of Hebrew Bel), name of Marduk.

It is identical with the first element in Beelzebub and the second in Hannibal ("grace of Baal") and Hasdrubal ("help of Baal"). The name has been used figuratively in English for any "false god."

1650s, "powerful in operation," from Greek energetikos "active," from energein "to work, be in action, act upon" (see energy). Of persons, "active," in English from 1796 (energetical "operative" is from c. 1600; from 1630s as "full of energy," while energical is attested from 1560s). Related: Energetically.

"pertaining to the radiometer or to experiments performed by it," 1877, from radiometer "instrument to transform radiant energy into mechanical work" (1875), radiometry, from radio-, here indicating "radiant energy," + -metric. Previously radiometer was the name of an old cross-staff instrument for measuring angles. Radiometric dating is attested from 1906.

1550s, "a number attached to a year to show the number of days into the calendar moon on which the solar year begins;" 1580s, "number of days by which the solar year exceeds a lunar one of 12 moons;" from French épacte (12c.), from Late Latin epacta "an intercalary day," from Greek epakte (plural epaktai, in epaktai hemerai "intercalary days"), from fem. of epaktos "brought in, imported, alien," verbal adjective of epagein "to add, bring forward," also "intercalate," from epi "on" (see epi-) + agein "put in motion, move; push forward, advance," from PIE root *ag- "to drive, draw out or forth, move." Related: Epactal.

a model solar-system machinery constructed to represent the motions of the planets in their orbits, 1713, invented c. 1704 by English clockmaker George Graham (1673-1751) and constructed by instrument maker John Rowley. Graham gave a copy to his patron, Charles Boyle (1674-1731), 4th Earl of Orrery (Cork) and named it in his honor.