From The Collaborative International Dictionary of English v.0.48 [gcide]:
Residuum \Re*sid"u*um\ (-?m), n. [L. See Residue.]
That which is left after any process of separation or
purification; that which remains after certain specified
deductions are made; residue.
[1913 Webster]
"I think so," is the whole residuum . . . after
evaporating the prodigious pretensions of the zealot
demagogue. --L. Taylor.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
residuum
n 1: something left after other parts have been taken away;
"there was no remainder"; "he threw away the rest"; "he
took what he wanted and I got the balance" [syn:
remainder, balance, residual, residue, residuum,
rest]