This word is used by Tor Nørretranders in his book The User Illusion, published in Danish in 1991 and in English in 1998. He argues that effective communication depends on a shared body of knowledge between the persons communicating. If someone is talking about cows, for example, what is said will be unintelligible unless the person listening has some idea what a cow is, what it is good for, and in what contexts one might encounter one. In using the word "cow", Nørretranders says, the speaker has deliberately thrown away a huge body of information, though it remains implied. He illustrates the point with a story of Victor Hugo writing to his publisher to ask how his most recent book, Les Miserables, was getting on. Hugo just wrote "?", to which his publisher replied "!", to indicate it was selling well. The exchange would have no meaning to a third party because the shared context is unique to those taking part in it. This shared context Tor Nørretranders calls exformation. He coined the word as a abbreviated form of explicitly discarded information, originally in Danish as eksformation; the word first appeared in English in an article he wrote in 1992. He says "exformation is everything we do not actually say but have in our heads when or before we say anything at all. Information is the measurable, demonstrable utterance we actually come out with".

From the information content of a message alone, there is no way of measuring how much exformation it contains.

[Tor Nørretranders, The User Illusion (1998)]

Thought, argues Norretranders, is in fact a process of chucking away information, and it is this detritus (happily labelled "exformation") that is crucially involved in "automatic" behaviours of expertise (riding a bicycle, playing the piano), and which is therefore the most precious to us as people.

[Guardian, Sep. 1998]