Etymology
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monoxide (n.)

"oxide with one oxygen atom in each molecule," 1840, from mono- "single" + oxide.

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interact (v.)
"act on each other, act reciprocally," 1805, from inter- + act (v.). Related: Interacted; interacting.
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duodenary (adj.)

"relating to the number twelve, twelve-fold," 1766, from Latin duodenarius "containing twelve," from duodeni "twelve each," from duodecim "twelve" (see dozen).

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cat-head (n.)

"beam projecting from each side of the bows of a ship to hold the anchor away from the body of the ship," 1620s, from cat (n.) in some obscure sense.

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interchangeable (adj.)
late 14c., entrechaungeable, "mutual, reciprocal," from inter- + changeable. Meaning "capable of being used in place of each other" is from 1560s. Related: Interchangeably.
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crossbones (n.)

also cross-bones, "figure of two thigh-bones laid across each other in the form of an X," 1798, from cross- + bone (n.).

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interactive (adj.)
"acting upon or influencing each other," 1832, from interact (v.), probably on model of active. Related: Interactively; interactivity.
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per annum 

"in each year, annually," c. 1600, Latin, "by the year," from per (see per) + annum, accusative singular of annus "year" (see annual (adj.)).

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backstitch (n.)
also back-stitch, 1610s, from back (adj.) + stitch (n.). So called because each stitch doubles back on the preceding one. As a verb from 1720.
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Fibonacci (adj.)
1891 in reference to a series of numbers in which each is equal to the sum of the preceding two, from name of Leonardo Fibonacci (fl. c. 1200) Tuscan mathematician.
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