From the second edition (1989):
deflexion, deflection
(dɪˈflɛkʃən) [ad. L. dēflexiōn-em, n. of action f. dēflectĕre (ppl. stem dēflex-) to deflect. Cf. mod.F. déflexion (Dict. Acad. 1762, occurring also in 16th c. as déflection). The non-etymological spelling deflection, now very common, is taken from the present-stem deflect-, associated with nouns of action from L. ppl. stem in -ect-, as collection, dissection, etc.]
1. The action of bending down; the condition of being bent or curved; also, a bend or curve (as a result).
In Mech. The bending of any body under a transverse strain; the amount of this. In Entom. The state of being bent downward, as the deflexion of the wings when folded; also, a deflected part or margin.
1665 Sir T. Herbert Trav. (1677) 296 The Mahometans signifie the same onely by a moderate deflexion of the head. 1821 Tredgold Ess. Cast Iron (1824) 73 When the weights were removed, the piece retained a permanent deflexion. 1879 Cassell's Techn. Educ. IV. 276/2 The deflection of a beam supporting a lateral weight.
2. The action of turning, or state of being turned, away from a straight line or regular path; the amount of such deviation; also, a turn or deviation (as an effect or result).
1665 Phil. Trans. I. 105 Of which deflection he ventures to assign the cause. 1831 Brewster Newton (1855) I. xii. 292 In 1684‥Newton discovered that the moon's deflexion in a minute was sixteen feet, the same as that of bodies at the earth's surface. 1833 Herschel Astron. viii. 267 Deflection from a straight line is only another word for curvature of path. 1862 Merivale Rom. Emp. (1865) VII. lxi. 329 They‥possibly noted the great deflection of the coast southward from Cape Wrath.
b. Of things immaterial.
1605 Bacon Adv. Learn. ii. i. §3 Of the works of nature which have a digression and deflexion from the ordinary course of generations, productions, and motions. 1648 W. Mountague Devout Ess. i. 112 (T.) King David found this deflection and indirectness in our minds. 1649 Jer. Taylor Gt. Exemp. ii. ix. 123 Deflexions in manners. 1840 De Quincey Style iii. Wks. X. 190 We shall point out the deflexion, the bias, which was impressed upon the Greek speculations in this particular. 1851 Carlyle Sterling i. xiv. (1872) 86 At this extreme point of spiritual deflexion and depression. 1876 Mozley Univ. Serm. iv. (1877) 84 The type of religion it has produced is a deflection from simplicity.
3. The turning of a word or phrase aside from its actual form, application, or grammatical use. arch.
1603 Holland Plutarch's Mor. 1311 By a little deflexion of the name‥that Canicular or Dogge starre is called Κυων. a1619 M. Fotherby Atheom. ii. i. §8 (1622) 191 That censure of Catullus (with a little deflection) might very fitly bee applied vnto him. 1659 O. Walker Oratory 34 By a gentle deflexion of the same word, in changing the substantive with the adjective. 1807 G. Chalmers Caledonia I. i. iv. 119 Grym signifies strength; and hence, by a little deflexion, Grym came to signify any strength. 1830 De Quincey Bentley Wks. 1890 IV. 131 note, A practice arose of giving to Greek names in as their real Greek termination, without any Roman deflexion.
4. Electr. and Magn. The turning of a magnetic needle away from its zero; the measured amount by which it is deflected.
1646 Sir T. Browne Pseud. Ep. ii. ii. 62 The variation of the compasse is‥a deflexion and siding East and West from the true meridian. 1863 Tyndall Heat i. 4 A moment's contact suffices to produce a prompt and energetic deflection of the needle. 1865 Pall Mall G. 3 Aug. 1/2 The curious electrical phenomenon known to electricians as ‘deflection’, has to-day been observed through the United Kingdom.
5. Optics. The bending of rays of light from the straight line. By Hooke applied specifically to the apparent bending or turning aside of the rays passing near the edge of an opaque body, called by Newton inflexion, and now explained as a phenomenon of diffraction.
(Brougham tried to differentiate inflexion and deflexion: see quot.)
1674–5 Hooke Lect. Light Wks. (1705) 188 The Light from the Edge [of a card or razor] did strike downwards into the Shadow very near to a Quadrant, though still I found, that the greater the Deflection of this new Light was from the direct Radiations of the Cone, the more faint they were. 1727–51 Chambers Cycl., Deflection of the Rays of Light, is a property which Dr. Hook observed 1674⁄5‥He says, he found it different both from reflexion, and refraction‥This is the same property which Sir Isaac Newton calls Inflection. 1796 Brougham in Phil. Trans. LXXXVI. 228 Def. 1. If a ray passes within a certain distance of any body, it is bent inwards; this we shall call Inflection. 2. If it passes at a still greater distance it is turned away; this may be termed Deflection. 1808 J. Webster Nat. Phil. 174 This deflection is supposed to proceed from the attraction of the denser medium. 1831 Brewster Newton viii. (1839) 99 In his paper of 1674‥he [Hooke]‥described the leading phenomena of the inflexion, or the deflexion of light, as he calls it.
6. Naut. The deviation of a ship from her true course in sailing.
1706 Phillips (ed. Kersey), Deflection‥In Navigation, the Tendency of a Ship from her true Course, by means of Currents, &c. which divert or turn her out of her right Way.