An unplanned expedient.

synonym: improvisation.

Roget 147: substitution, commutation; supplanting etc. v.; metaphor, metonymy etc. (figure of speech) 521.    [Thing substituted] substitute, ersatz, makeshift, temporary expedient, replacement, succedaneum; shift, pis aller [Fr.], stopgap, jury rigging, jury mast, locum tenens, warming pan, dummy, scapegoat; double; changeling; quid pro quo, alternative.    representative etc. (deputy) 759; palimpsest.    price, purchase money, consideration, equivalent.    ... show more