A volte-face is a complete reversal of position, opinion, or policy. The term comes to English via French volte face, ultimately from Italian voltafaccia from voltare ("to turn") and faccia ("face").
It is most often used in the field of politics, to refer to a politician's completely reversing their position merely for sake political expediency. The term has a strongly negative connotation, and conveys a feeling of betrayal. A similar term in contemporary politics is "flip-flop."