Definition of caustic

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caustic potash >

caus·tic    (kôstk) KEY 

ADJECTIVE:

1. Capable of burning, corroding, dissolving, or eating away by chemical action.
2. Corrosive and bitingly trenchant; cutting. See Synonyms at sarcastic.
3. Causing a burning or stinging sensation, as from intense emotion: "Most of all, there is caustic shame for my own stupidity" (Scott Turow).

NOUN:

1. A caustic material or substance.
2. A hydroxide of a light metal.
3. The enveloping surface formed by light rays reflecting or refracting from a curved surface, especially one with spherical aberration.

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ETYMOLOGY:
Middle English caustik, from Latin causticus, from Greek kaustikos, from kaustos, from kaiein, kau-, to burn

OTHER FORMS:
causti·cal·ly(Adverb), caus·tici·ty  (kô-sts-t) KEY (Noun)