na-adjective
Formal, somewhat literary word used to describe something that is probable or likely based on reasoning or evidence. Often used in academic or analytical contexts.
この仮説は蓋然的な推論に基づいている。
This hypothesis is based on probable reasoning.
彼の成功
It can be said that his success is likely.
ありうる is a more common and casual way to express possibility, whereas 蓋然的 is formal and emphasizes reasoned likelihood.
From 蓋然 (がいぜん, 'probability') + 的 (てき, adjectival suffix). 蓋然 is a Sino-Japanese word composed of 蓋 (cover, probably) and 然 (state, condition).