The Collaborative International Dictionary of English v.0.48:

Omnium \Om"ni*um\, n. [L., of all, gen. pl. of omnis all.] (Eng.
   Stock Exchange)
   The aggregate value of the different stocks in which a loan
   to government is now usually funded. --M'Culloch.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

OMNIUM, mercant. law. A term used to express the aggregate value of the
different stocks in which a loan is usually funded. 2 Esp. Rep. 361; 7 T. R.
630.