1Nature — the natural world and its environment (mountains, rivers, plants, animals, etc.), not shaped by human hands.
日本の自然は、四季がはっきりしていて美しい。
Japan's nature is beautiful, with clearly defined four seasons.
山の自然を守るために、清掃ボランティアに参加した。
I took part in a cleanup volunteer program to help protect the mountain's natural environment.
自然の中でキャンプをすると、心が落ち着く。
Camping out in nature calms the mind.
2Unforced and genuine, not artificial or contrived; occurring or appearing spontaneously and effortlessly.
彼の笑顔は、とても自然だ。
His smile looks very natural.
演技が上手で、話し方もごく自然に聞こえる。
The acting is skillful, and the way he speaks sounds completely natural.
その反応は、作られたものではなく自然なものだった。
That reaction wasn't staged; it was genuine.
3Happening by itself without deliberate effort; commonly used in the forms 自然に or 自然と to mean "naturally" or "of its own accord."
話しているうちに、自然と笑顔になった。
While we were talking, I found myself smiling without meaning to.
問題は、時間が経てば自然に解決することもある。
Some problems resolve themselves naturally with time.
毎日日本語に触れていれば、単語は自然に覚えられる。
If you're exposed to Japanese every day, you'll naturally pick up new words.