As an adjective, constant means happening all the time or staying the same. As a noun, it refers to something that does not change, especially in math or science.
From Latin *constans*, present participle of *constare* "to stand firm" (*con-* "together" + *stare* "to stand"). It has long meant something steady and unchanging.
A constant is like the fixed background in a changing scene. In math and physics, constants—like the speed of light—are the universe’s promises that some things won’t randomly shift on you.
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