na-adjective
self-contained; self-absorbed
Describes a person, system, or work that is complete in itself, not relying on or engaging with external elements. The 'self-absorbed' nuance applies when referring to a person who is wrapped up in their own world.
彼の小説は自己完結的な世界観を持っている。
His novel has a self-contained worldview.
彼女は自己完結的で、他人の意見をあまり聞かない。
She is self-absorbed and doesn't listen much to others' opinions.
Compound of 自己 (self) + 完結 (completion) + 的 (-like, -ish). The exact origin is uncertain, but it follows a common pattern for forming adjectival nouns.