a feature (quantity or property or function) that remains unchanged when a particular transformation is applied to it
"The length of an object in spacetime is invariant under Lorentz transformations, even though its measured distance changes depending on the observer's speed."
An invariant quantity, function etc.
"The energy of an isolated system remains invariant regardless of the observer's frame of reference."
In plain English: An invariant is something that stays exactly the same even when everything else around it changes.
"The invariant was the single value that remained unchanged regardless of how we transformed the data."
unaffected by a designated operation or transformation
"The geometric property of distance remains invariant under rotation and translation."
unvarying in nature
"maintained a constant temperature"
"principles of unvarying validity"
not varying; constant
"The speed of light remains invariant regardless of how fast an observer moves."
The word invariant comes from combining the prefix in- with variant to mean something that does not vary or change. It entered English as a direct formation describing qualities that remain constant despite other changes.