reflex

Etymology

From Late Latin reflexus, past participle of reflectere (“to bend back”). Photography sense is from noun sense meaning “reflection”.

noun

1. An automatic response to a simple stimulus which does not require mental processing.
   For a while, I shall have to make a conscious effort to smile, nod, stand and perform the thousands of little gestures which constitute life on Earth, and then those gestures will become reflexes again. 1970, Stanisław Lem, trans. Joanna Kilmartin and Steve Cox, Solaris
   He met Luis Suarez's cross at the far post, only for Chelsea keeper Petr Cech to show brilliant reflexes to deflect his header on to the bar. Carroll turned away to lead Liverpool's insistent protests that the ball had crossed the line but referee Phil Dowd and assistant referee Andrew Garratt waved play on, with even a succession of replays proving inconclusive. May 5, 2012, Phil McNulty, “Chelsea 2-1 Liverpool”, in BBC Sport
2. (linguistics) The descendant of an earlier language element, such as a word or phoneme, in a daughter language.
   Coordinate term: cognate
3. (linguistics, rare) The ancestor word corresponding to a descendant.
   The Middle Korean reflex for mey was mwoy 뫼 (mwoy) 2011, Ki-Moon Lee, S. Robert Ramsey, A history of the Korean language
4. The descendant of anything from an earlier time, such as a cultural myth.
   The superstition of the loup-garou, or werewolf, belongs to the folklore of most modern nations, and has its reflex in the story of "Little Red Riding-hood" and others. 1898, Christian Brinton, The Century
5. (chiefly photography) Reflection or an image produced by reflection. The light reflected from an illuminated surface to one in shade.
   A reflex camera uses a mirror to reflect the image onto a ground-glass viewfinder.
   Lucy is sleeping soundly; the reflex of the dawn is high and far over the sea[.] 1897, Bram Stoker, Dracula, published 1993, page 88

adj

1. Bent, turned back or reflected.
2. Produced automatically by a stimulus.
3. (geometry, of an angle) Having greater than 180 degrees but less than 360 degrees.
   A polygon is said to be convex when no one of its angles is reflex. 1878, James Maurice Wilson, Elementary Geometry, MacMillan, page 10
   An angle less than a right angle is said to be acute; one greater than a right angle but less than a straight angle is said to be obtuse; one greater than a straight angle but less than a perigon is said to be reflex or convex. 1895, David Eugen Smith, Wooster Woodruff Bernan, New Plane and Solid Geometry, page 7
   If the reflex region is the interior of the angle, the dihedral angle is reflex. 1958, Howard Fehr, “On Teaching Dihedral Angle and Steradian” in The Mathematics Teacher, v 51, National Council of Teachers of Mathematics, page 275
   A reflex edge of a polyhedron is an edge where the inner dihedral angle subtended by two incident faces is greater than 180°. 1991, B. Falcidieno et al., “Configurable Representations in Feature-based Modelling”, in Eurographics '91: Proceedings, North-Holland, page 145
   We say that an angle is convex if it is not reflex. 2001, Esther M. Arkin et al., “On the Reflexivity of Point Sets”, in Algorithms and data structures: 7th International Workshop, WADS 2001: Proceedings, Springer, page 195
   P denotes a polygon and r the number of reflex vertices. 2004, Ana Paula Tomás and António Leslie Bajuelos, “Quadratic-Time Linear-Space Algorithms Generating Orthogonal Polygons with a Given Number of Vertices”, in Computational Science and Its Applications – ICCSA 2004 Proceedings, part 3, Springer, page 117
4. (painting) Illuminated by light reflected from another part of the same picture.

verb

1. (transitive) To bend back or turn back over itself.
2. (transitive, obsolete) To reflect (light, sight, etc.).
3. (transitive, obsolete) To reflect or mirror (an object), to show the image of.
4. (transitive, obsolete) To cast (beams of light) on something.
5. To respond to a stimulus.