An Etymological Dictionary of Astronomy and Astrophysics

1. Literary works invented by the imagination, such as novels or short stories.
   

2. An invented story or explanation; lie.
   

3. The act of inventing a story or explanation.
   

4. Law: Something assumed to be true for the sake of convenience, though probably false (Dictionary.com).

Etymology (EN): M.E., from O.Fr. ficcion “dissimulation, ruse; invention, fabrication” and directly from L. fictionem “a fashioning or feigning,” noun of action from p.p. stem of fingere “to shape, form, devise, feign,” originally “to knead, form out of clay,” from PIE *dheigh- “to build, form, knead;” akin to Skt. dehah “body,” literally “that which is formed,” dih- “to besmear;” Gk. teikhos “wall;” L. fingere “to form, fashion,” Gothic deigan “to smear;” O.Irish digen “firm, solid.”

Etymology (PE): Formed on the model of fiction, as above, from diz- “to build, to form;” (related to Pers. dež, dez “fortress”);
cf. Mid.Pers. dys-/dēs- “to build;” Sogd. dys “to build;” Av. (+ *pari-) daēz- “to build (around);” Proto-Ir. *daiz- “to build, form;” from PIE *dheigh- “to build, form,” as above, + suffix -an.