1. something capable of working successfully
the project was/wasn't viable
Something described as 'viable' is feasible, possible or capable of being used or working correctly. We can talk about 'viable plans' - plans that are likely to be successful, or something that is 'economically viable' or capable of making a profit. When we are talking about plants a 'viable seed' is one that is likely to produce a plant or to germinate. In biology, something 'viable' is able to live or thrive successfully. This is a technical term used more often in professional contexts.