n. 1 a state of extreme physical or mental fatigue: he was pale with exhaustion. 2 the action or state of using something up or of being used up completely: the rapid exhaustion of fossil fuel reserves. - the action of exploring a subject or options so fully that there is nothing further to be said or discovered: the total exhaustion of viable systematic alternatives. - [LOGIC] the process of establishing a conclusion by eliminating all the alternatives. early 17th cent.: from late Latin exhaustio(n-),