It's been defined as:
"Temporarily and willingly setting aside our beliefs about reality in order to enjoy the make-believe of a play, a poem, film, or a story. Perfectly intelligent readers can enjoy tall-tales about Pecos Bill roping a whirlwind, or vampires invading a small town in Maine, or frightening alternative histories in which Hitler wins World War II, without being 'gullible' or 'childish.' To do so, however, the audience members must set aside their sense of 'what's real' for the duration of the play, or the movie, or the book.I think one reason that some people declare, vehemently, that they don't read fiction is either the desire to not appear "gullible" or "childish;" or an inability (or unwillingness) to temporarily set aside the capacity for distinguishing between the real and the unreal.
"Samuel Coleridge coined the English phrase in Chapter 14 of Biographia Literaria to describe the way a reader is implicitly 'asked' to set aside his notions of reality and accept the dramatic conventions of the theater and stage or other fictional work. Coleridge writes:" . . . My endeavours should be directed to persons and characters supernatural, or at least romantic; yet so as to transfer from our inward nature a human interest and a semblance of truth sufficient to procure for these shadows of imagination that willing suspension of disbelief for the moment, which constitutes poetic faith (quoted in Cuddon, page 1044)."Coleridge may have been inspired by the French phrase, 'cette belle suspension d'esprit de law sceptique' from François de La Mothe le Vayer, or by Ben Jonson's writing where Jonson notes, 'To many things a man should owe but a temporary belief, and suspension of his own judgment.' "
("Literary Terms and Definitions: W," Carson-Newman College, Jefferson City, Tennessee)
I know that quite a few perfectly reasonable and intelligent people don't read fiction. Including some of my relatives. I've no problem with that. I'd no more insist that everyone have my tastes in reading, than assume that people in general should enjoy the same leisure pursuits that I do.