na-adjective
Formal or literary term for something that is not clearly defined, known, or distinguishable. Often used in written or academic contexts.
彼の説明は不分明で、何を言いたいのかよくわからなかった。
His explanation was vague, and I couldn't really understand what he wanted to say.
この地図では国境
On this map, the national borders are unclear.
曖昧 is the everyday word for 'vague' or 'ambiguous'. 不分明 is more formal and less common, often used in written language.
Compound of 不 (negative prefix) and 分明 (clear, distinct). The exact historical derivation is uncertain, but the word follows a standard Sino-Japanese pattern for forming negative abstract nouns.