An argument with an unstated premiss or an unstated conclusion. This accords with the seventeenth-century definition of an enthymeme as ‘a syllogism complete in the mind and incomplete in expression,’ e.g., ‘If it is raining, I will take my umbrella; therefore, I will take my umbrella.’
Here, the premiss ‘It is raining’ is not stated, perhaps because on the particular occasion it will be too obvious for words.
In some enthymemes, it is the conclusion that is not explicitly stated, and again the reason may be that it is obvious. For instance: ‘It is raining, and if it is raining I will take my umbrella.’ At that point enough has been said for the purposes of normal conversation.
It may happen that both the conclusion and some premiss is implicit. The statement: ‘Either he is a rogue, or I will eat my hat’ can be understood as an enthymeme with a suppressed premise: ‘I will not eat my heat’ and a suppressed conclusion: ‘He is a rogue.’
— The Penguin Dictionary of Philosophy (Penguin Books, 2005), 189-190.