na-adjective
lukewarm; tepid; halfhearted; indifferent
Describes something that is neither hot nor cold, or a person's attitude that lacks enthusiasm or commitment. Often used in a negative sense to criticize a lack of passion or decisiveness.
彼の微温的な態度にはがっかりした。
I was disappointed by his halfhearted attitude.
微温的なお茶は美味しく
Lukewarm tea doesn't taste good.
ぬるい is a simpler i-adjective meaning 'lukewarm' (temperature), without the figurative sense of 'halfhearted'.
Compound of 微温 (びおん, 'lukewarm') + 的 (てき, '-like, -ish'). The exact historical derivation is uncertain, but the word follows a common pattern of forming na-adjectives from nouns with 的.