The Collaborative International Dictionary of English v.0.48:

Apportion \Ap*por"tion\, v. t. [imp. & p. p. Apportioned; p.
   pr. & vb. n. Apportioning.] [OF. apportionner, LL.
   apportionare, fr. L. ad + portio. See Portion.]
   To divide and assign in just proportion; to divide and
   distribute proportionally; to portion out; to allot; as, to
   apportion undivided rights; to apportion time among various
   employments.
   [1913 Webster]

WordNet (r) 3.0 (2006):

apportioning
    n 1: the act of distributing by allotting or apportioning;
         distribution according to a plan; "the apportionment of
         seats in the House of Representatives is based on the
         relative population of each state" [syn: allotment,
         apportionment, apportioning, allocation, parceling,
         parcelling, assignation]