Once; formerly; previously — indicates that something was true at some time in the past.
この場所はかつて湖だったと言われている。
It is said that this place was once a lake.
彼はかつて有名な俳優だった。
He was once a famous actor.
かつてこの町は栄えていた。
This town used to prosper.
Never yet; never before — used with a negative verb to mean that something has not happened up to now.
かつて彼に会ったことはない。
I have never met him.
この問題はかつて解決されたことがない。
This problem has never been solved before.