1The state of being unaltered by human intervention; naturally occurring. Often used to modify a noun, as in "natural material."
このジュースは天然の果汁だけで作られている。
This juice is made only from natural fruit juice.
天然のダイヤモンドは人工のものより高価だ。
Natural diamonds are more expensive than artificial ones.
この地域は天然資源が豊富だ。
This region is rich in natural resources.
天然の温泉に浸かると疲れが取れる。
Soaking in a natural hot spring relieves fatigue.
2A colloquial term (short for 天然ボケ) for someone who is innocently scatterbrained or ditzy, usually said affectionately.
彼女は天然で、みんなを笑わせてしまう。
She's a natural airhead and always ends up making everyone laugh.
天然な性格のおかげで、彼はクラスの人気者だ。
Thanks to his airheaded personality, he's popular in class.