He stayed there for four years, a time he has described as a major inflexion point in his life.
British Dictionary definitions for inflexion
inflection
/ɪnˈflɛkʃən/
noun
1.
modulation of the voice
2.
(grammar) a change in the form of a word, usually modification or affixation, signalling change in such grammatical functions as tense, voice, mood, person, gender, number, or case
3.
an angle or bend
4.
the act of inflecting or the state of being inflected
5.
(maths) a change in curvature from concave to convex or vice versa See also point of inflection