Lie: Conscious assertion of what the speaker knows he does not believe.
Note that it is not required that a lie is a false statement: What is required is that its speaker considers it one, but states it as if it is true.
Most human lying is in fact done by the conscious non-saying of truths one does know but rather does not give voice to in public, whether from cowardice or self-interest. A large part of public lying - as in the tale of the emperor's clothes - is collective collaborative public non-saying of things, that may indeed be motivated by justified self-interest, as in dictatorships, or common politeness, but also by conformist egoism.
That what is a lie for the speaker may be a truth in fact was remarked above, and it needs also remarking that there is a considerable difference between the lying of children and that of adults: Children tend to really mean what they say, if they don't lie, but most adults have falsified themselves and pretended so much that they are hardly capable, in many circumstances, to clearly distinguish their poses and pretensions of what they think they should socially seem to be and what they could know is true or likely.
There also is a considerable difference between the lying of children and of adults: Children really mean things, whereas most adults have falsified and pretended so much that they are hardly capable of real and authentic feeling and thinking, since they need falsified versions of themselves to exist socially.
That is, the lies of children are conscious; the lies of grownups are usually semi- conscious: They are playing some role and produce the required inauthenticity and falsehoods semi automatically "because that is the proper thing to do". And as remarked, much lying consists of ommissions to say what one really thinks (while knowing someone else, who might have been told the truth by the tacit person, has a personal concern or danger in not knowing the truth).
The majority of human beings is not much interested nor finds much profitable in true or probable ideas, but is much interested in pleasant illusions and profitable lies.
And the majority knows this very well, although they rarely admit it in public.
And the sad fact concerning lying - see Features of Moral Norms - is that the majority of human beings let themselves be deceived by lies they could rather easily have seen through, if only the lies they accept as if it were truths are the lies of their own leaders, or are the lies which support their own desires or delusions - which they then support with great moral pride as the socially and morally decent and moral thing to say and believe.