Etymology
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tesseract (n.)
"four-dimensional 'cube,'" 1888, from tessera + Greek aktis "ray" (see actino-).
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moonbeam (n.)

"ray of light from the moon," 1580s, from moon (n.) + beam (n.).

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radiotherapy (n.)

"treatment of disease by means of x-rays," 1902, from radio- + therapy.

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sea monkey (n.)
1909 as a heraldic animal, 1964 as a U.S. proprietary name for brine shrimp (Artemia salina), which had been used as food for aquarium fish till they began to be marketed as pets by U.S. inventor Harold von Braunhut (d.2003), who also invented "X-Ray Specs" and popularized pet hermit crabs. He began marketing them in comic book advertisements in 1960 as "Instant Life," and changed the name to Sea Monkeys in 1964, so called for their long tails.
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radio- 
word-forming element meaning 1. "ray, ray-like" (see radius); 2. "radial, radially" (see radial (adj.)); 3. "by means of radiant energy" (see radiate (v.)); 4. "radioactive" (see radioactive); 5. "by radio" (see radio (n.)).
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decussate (v.)

"to intersect so as to form a figure like the letter X, to cross," 1650s, from Latin decussatus, past participle of decussare "to divide crosswise, to cross in the form of an 'X,'" from decussis "the figure 'ten'" (in Roman numerals, represented by X), also "a large copper coin ten times the value of an as," from decem "ten" (from PIE root *dekm- "ten"). Related: Decussated; decussating; decussation. As an adjective, by 1806.

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radioscopy (n.)

"examination by means of x-rays," 1896, from radio- + -scopy.

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actino- 
before vowels actin-, word-forming element meaning "pertaining to rays," from Latinized form of Greek aktis (genitive aktinos) "ray of light, beam of light; spoke of a wheel;" of unknown etymology. It is perhaps cognate with Sanskrit aktuh "light, ray," Gothic uhtwo "dawn, daybreak," Lithuanian anksti "early."
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roentgen 

in physics, 1896, in Roentgen rays "X-rays," in recognition of German physicist Wilhem Conrad Röntgen (1845-1923), who discovered X-rays in 1895. As a unit of exposure to radiation, it is attested from 1922, proposed in French in 1921.

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mammography (n.)

"examination of the breast by means of X-rays," by 1937, from mammo- "breast" + -graphy.

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