na-adjective, noun
unexperienced; never before felt; first-time
Describes a sensation, event, or flavor that one has never experienced before. Often used with nouns like 感覚, 味, or 出来事.
それは未体験の味だった。
It was a flavor I had never experienced before.
未体験の感覚に戸惑った。
I was bewildered by the sensation I had never felt before.
初めて is an adverb meaning 'for the first time', whereas 未体験 is an adjectival noun describing the state of being unexperienced.
Compound of 未 (mi, 'not yet') and 体験 (taiken, 'experience').