From The Collaborative International Dictionary of English v.0.48 :
out-migration \out-migration\ n.
migration out of country or territory; emigration.
Syn: emigration.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) :
out-migration
n 1: migration from a place (especially migration from your
native country in order to settle in another) [syn:
emigration, out-migration, expatriation]