na-adjective
Describes a view, argument, or judgment that considers only one aspect and lacks balance. Often used critically.
彼の意見は一面的で、他の視点を無視している。
His opinion is one-sided and ignores other perspectives.
A one-sided view won't reveal the essence of the problem.
Antonym: 多面的 means multifaceted or considering many aspects.
Compound of 一面 (one side, one aspect) + 的 (-like, -ish). The exact historical derivation is uncertain, but the word follows a common pattern for forming adjectival nouns.