What is an invariable quantity; specifically, a function of the coefficients of one or more forms, which remains unaltered, when these undergo suitable linear transformations. J. J. Sylvester called?

invariant — An invariable quantity; specifically, a function of the coefficients of one or more forms, which remains unaltered, when these undergo suitable linear transformations. J. J. Sylvester. (Webster's 1913.)

Sources