= Marginal utility
{title2=$u'(x)$}
{wiki}
The additional <utility function> obtained per additional unit of <consumption> or <portfolio wealth>. For a differentiable <utility function> it is its <derivative>; for a <concave> <utility function> it is nonincreasing. In a complete-market allocation, equating marginal utility to a multiplier times the <state-price density> compares the utility benefit of a payoff with its financing cost. The <concave supporting-tangent inequality> makes this comparison a global optimality certificate rather than only a local first-order condition.
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