There are different ways to lie. The classical definition is ‘to enunciate a falsehood with the intent to deceive’, derived from Saint Augustine, adopted by Saint Thomas (I-II, 110, 1-2), who draws from this the criteria for a full-blown lie: Falsehood in what is spoken, awareness that such is false and the deception.
There can be lesser lies, lies without the intent to deceive, for one may even speak what is false without being aware one is doing so. There are omissions, hiding the truth – or what is often worse, some portions of the truth, when it should be revealed. Then there are all the rationalizations, the vague sense that what one is saying might be untrue, but one is so committed, or so coerced – as were the crowds applauding the disrobed emperor – one is loath to speak the truth that is so obvious.
Then are the various intentions for telling a lie distinguished by Thomas. Mischievous – to deliberately harm another; officious – lies told for a good reason; and jocose – to give some sort of pleasure.