noun which may take the genitive case particle 'no', na-adjective
Describes a very high degree or intensity, often used with nouns like 緊張, 疲労, or 状態. Functions as a no-adjective or na-adjective.
彼は極度の緊張で声が出なかった。
He was so extremely nervous that he couldn't speak.
極度に疲れているときは、無理をしない
When you are extremely tired, it's better not to push yourself.
Compound of 極 (きょく, 'extreme, pole') and 度 (ど, 'degree'). The meaning is transparent from its components.