Caustique

Caustique is a French term used to describe something that burns or corrodes, either literally or figuratively. In English, the corresponding term is caustic. The word covers several domains: chemistry (caustic substances), optics and geometry (caustics produced by light rays), and rhetoric (caustic humor).

In chemistry, caustics are highly basic and strongly corrosive substances capable of damaging biological tissue. Common examples are sodium hydroxide (caustic soda) and potassium hydroxide (caustic potash). They are widely used in soap making, chemical manufacturing, paper production, and drain cleaning. Handling caustics requires protective equipment and proper storage due to their exothermic reactions and potential for severe burns. The term distinguishes these substances from milder alkalis.

In optics, a caustic is the envelope of light rays reflected or refracted by a curved surface, resulting in bright curves or patterns. Examples include the luminous curves at the bottom of a coffee cup, the shimmering edges inside a glass of water, or the characteristic patterns seen in architectural lighting. In mathematics and geometry, caustics are studied as envelopes of families of rays and have applications in optical design and computer graphics.

In rhetoric, caustic describes language that is cutting, biting, or severely critical. A caustic remark aims to wound or puncture through sarcasm or irony.

The term derives from Latin causticus, from Greek kaustikos meaning burning.