In informal style, when is often used after be in definitions: A dilemma is when you don’t know which way to turn. The construction is useful, but it is widely regarded as incorrect or as unsuitable for formal discourse. In formal style, there is no alternative but to rephrase such definitions to avoid is when. The trick here is to make the first part of the sentence a full clause: A dilemma is a situation in which you don’t know which way to turn. You are in a dilemma when you don’t know which way to turn.
