(1) Capable of working successfully; feasible. (Oxford English Dictionary)
(3) capable of living; especially : having attained such form and development as to be normally capable of surviving outside the mother's womb. (Oxford English Dictionary)
(3) A viable system is any system organised in such a way as to meet the demands of surviving in the changing environment. (Beer 1967)
(1) & (2) Oxford English Dictionary, 2013., s.v. "viable."
(3) Beer, S. 1967. Cybernetics and Management, 2nd edition. London, UK: English Universities Press.
(1) and (2) are dictionary definitons which illustrate the two basic ideas of able to do its job, and have reached sufficent maturity to be able to survive in its environment.
(3) Is taken from Beer's Viable Systems Model, which describes the things an organization needs to be viable.