also: いちてんき
noun
Refers to a decisive moment or event that changes the course of a situation, similar to 転機 but with a slightly more formal or literary feel.
あの出来事が彼の人生の一転機となった。
That event became a turning point in his life.
この発見は科学史の一転機
This discovery can be called a turning point in the history of science.
転機 is the more common and everyday word for 'turning point'. 一転機 is less frequent and can sound slightly more emphatic or formal.
Compound of 一 (one) and 転機 (turning point), literally 'one turning point'. The exact historical derivation is uncertain, but it likely emphasizes a single, decisive change.