Etymology
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magnitude (n.)

c. 1400, "pre-eminence, magnificence;" early 15c., "greatness of size or extent," from Latin magnitudo "greatness, bulk, size," from magnus "great" (from suffixed form of PIE root *meg- "great") + -tudo, suffix forming abstract nouns from adjectives and participles (see -tude).

Meaning "size, extent," whether great or small is from early 15c. Of stars, "brightness or brilliancy expressed as a number" (now on a logarithmic scale) from 1640s, translating Ptolemy's Greek megethos.

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magnitudinous (adj.)

"involving greatness of scale," 1789; see magnitude + -ous.

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*meg- 
Proto-Indo-European root meaning "great."

It forms all or part of: acromegaly; Almagest; Charlemagne; maestro; magisterial; magistral; magistrate; Magna Carta; magnate; magnitude; magnum; magnanimity; magnanimous; magni-; Magnificat; magnificence; magnificent; magnify; magniloquence; magniloquent; Magnus; maharajah; maharishi; mahatma; Mahayana; Maia; majesty; major; major-domo; majority; majuscule; master; maxim; maximum; may (v.2) "to take part in May Day festivities;" May; mayor; mega-; megalo-; mickle; Mister; mistral; mistress; much; omega.

It is the hypothetical source of/evidence for its existence is provided by: Armenian mets "great;" Sanskrit mahat- "great, mazah- "greatness;" Avestan mazant- "great;" Hittite mekkish "great, large;" Greek megas "great, large;" Latin magnus "great, large, much, abundant," major "greater," maximus "greatest;" Middle Irish mag, maignech "great, large;" Middle Welsh meith "long, great."
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percentile (n.)

in statistics, "each of a series of values obtained by dividing a large number of quantities into 100 equal groups in order of magnitude," 1885, coined by English scientist Francis Galton (1822-1911) from percent + -ile.

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Richter 

in reference to the logarithmic scale for expressing magnitude of earthquakes, 1938, named for its deviser, U.S. seismologist Charles Francis Richter (1900-1985). Figurative use by 1967.

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luminosity (n.)
1630s, "quality of being luminous," from French luminosité (cognate with Medieval Latin luminositas "splendor") or else a native formation from luminous + -ity. Meaning "intensity of light in a color" (of a flame, spectrum, etc.) is from 1876. In astronomy, "intrinsic brightness of a heavenly body" (as distinguished from apparent magnitude, which diminishes with distance), attested from 1906.
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apparent (adj.)
late 14c., "indisputable, clearly understood;" c. 1400, "easily seen or perceived," from Old French aparant "evident, obvious, visible," from Latin apparentem (nominative apparens) "visible, manifest," present participle of apparere "appear, come in sight" (see appear).

First attested in phrases such as heir apparent (see heir). Meaning "superficial, spurious" is from c. 1400; that of "appearing to the senses or mind but not necessarily real" is from 1640s. Apparent magnitude in astronomy (how bright a heavenly body looks from earth, as opposed to absolute magnitude, which is how bright it really is) is attested from 1875. Middle English had noun forms apparence, apparency, but both are obsolete from 17c.
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grandeur (n.)

c. 1500, "loftiness, height," from French grandeur, from Old French grandor "size, height, extent, magnitude; greatness" (12c.), from grand "great" (see grand (adj.)). "Being a word of late adoption, it retains the Fr. form -eur of the suffix." Extended sense of "majesty, stateliness" in English is first recorded 1660s.

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no-smoking (adj.)

"in which smoking is not permitted," 1905; the sign wording itself is attested by 1817.

Smoking is a vice to [sic] — and a national one, of such magnitude that railroad corporations throughout all their routes in the United States, have a special command in large letters, conspicuously placed at depots and inside of the cars — "No smoking allowed here." ["The Sailor's Magazine," December 1840]
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degradation (n.)

1530s, "a reduction in rank or dignity," from French dégradation (14c., Old French degradacion), noun of action from past-participle stem of degrader (see degrade). From 1752 as "state of being reduced from a higher to a lower grade or power;" by 1769 as "reduction of strength, value, magnitude, etc." Related: Degradational.

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