brain drain

Short definition from Wiktionary

Noun

1. 1
   The migration of educated or talented people from less economically advanced areas to more economically advanced areas, especially to large cities or more developed nations.
2. 2
   A Jackson-Pratt drain.

Verb

1. 1
   Of a nation or society, to be deprived of educated or talented people, who in turn migrate to larger cities or more developed nations.
2. 2
   To become mentally or psychologically fatigued or exhausted.
3. 3
   Alternative form of brain drain.