proliferation

Short definition from Wiktionary

Noun

1. 1
   The process by which an organism produces others of its kind; breeding, propagation, procreation, reproduction.
2. 2
   The act of increasing, rising, or proliferating; augmentation, amplification, enlargement, escalation, aggrandizement.
3. 3
   The act of increasing, rising, or proliferating; augmentation, amplification, enlargement, escalation, aggrandizement: In paticular, the spread (proliferating) of biochemical, nuclear and other weapons of mass destruction to countries not originally involved in developing them.
4. 4
   The result of building up; buildup, accretion.