An adverb meaning 'never (before)'; typically used with a negative verb to say that something has never occurred.
彼が遅刻したことは未だかつて一度もない。
He has never been late even once.
未だかつてこれほど大きな台風は記録されていない。
A typhoon this strong has never been recorded before.
私は未だかつてこんなに感動したことはない。
I have never been this moved before.