Viable (glossary)

(1) Capable of working successfully; feasible.

(3) capable of living; especially : having attained such form and development as to be normally capable of surviving outside the mother's womb

(3) A viable system is any system organised in such a way as to meet the demands of surviving in the changing environment.

Source(s)

(1) & (2) Oxford English Dictionary

(3) Beer, S. 1959. Cybernetics and Management. English Universities, London; Wiley, New York.

Discussion

(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 organisation needs to be viable.