noun which may take the genitive case particle 'no', noun
first in history; unprecedented
Used to describe something that has never happened before in recorded history. Often modifies nouns with の.
これは史上初の有人火星着陸だ。
This is the first manned landing on Mars in history.
史上初の快挙を成し遂げた。
They achieved an unprecedented feat.
前例のない means 'unprecedented' in a broader sense, not necessarily limited to historical firsts. 史上初 specifically claims a historical first.
Compound of 史上 (しじょう, 'in history') and 初 (はつ, 'first'). The reading はつ is the native Japanese reading for 初.