<p>Consider a decision maker who prefers to receive a reward when event <i>A</i> happens rather than—when event <i>B</i> happens, and who also prefers to receive the same reward when event <i>A</i> does not happen rather than—when event <i>B</i> does not happen. We define such decision maker as ambiguity averse (seeking) if event <i>B</i> (<i>A</i>) is unambiguous, i.e. measurable with objective probability. Under transitivity of preferences and a weak form of the first-order stochastic dominance, the proposed definition of ambiguity aversion (seeking) implies subadditivity (superadditivity) of matching probabilities of an ambiguous event and its complement. Implications for biseparable preferences (special cases of which include subjective expected utility theory, Choquet expected utility, multiple priors or maxmin expected utility, and α-maxmin theory) are provided. Ambiguity averse decision makers are willing to purchase full insurance for an uncertain event at a range of prices that are not necessarily actuarily fair. Ambiguity seeking decision makers never insure in full against an uncertain event. Our proposed definitions of ambiguity aversion (seeking) are unrelated to the concepts of smooth ambiguity aversion (seeking) undersmooth ambiguity model. An extension to fully uncertain situation (Anscombe-Aumann acts) is provided.</p>