What exactly is a lie?

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Is it anything we say which we know is untrue? Or something more than that? For example,suppose a friend wants to borrow some money from you. You say'I wish I could help you but I don't have enough money myself.' In fact,you are not short of money but your friend is in the habit of not paying his debts and you don't want to hurt his feelings by reminding him of this. Is this nearly a lie?