That is a very practical definition for economic models but that doesn't make it complete. Your link says value within a market, and a single offer for a company by Google does not automatically qualify as a market and therefor determine value. If it did, I could pay you 1 billion dollars for a rock and it would be "worth" 1 billion dollars. I prefer to think of it as, given imperfect information and an inability to predict the future, market value trends towards true value, where true value is a fuzzy subjective set of ideals held by individuals and society.
