In general, the opposite of trivial.
Proper
See also
Proper Divisor, Proper Ideal, Proper Subset, Proper Class, Proper Submodule, Proper Fraction, Proper Subfield, Proper Extension, Proper Superset, TrivialCite this as:
Weisstein, Eric W. "Proper." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Proper.html