turning point 
noun
A time at which a decisive change in a situation occurs, especially one with beneficial results: this could be the turning point in Nigel’s career...
- The turning points have been the result not of an accumulation of gradual changes in education or ‘enlightened’ official policy.
- All of the decisive turning points of German history were characterised by violent and usually bloody conflicts between the classes.
- Hardly any attention is given to the decisive turning points at which history might have taken a different path.
Synonyms
watershed, critical moment, decisive moment, crossroads, crisis, climacteric, moment of truth;
landmark