The plain (non-polite) style of speech or writing in Japanese, corresponding to forms like 'だ/である' and dictionary-form verbs (often contrasted with the polite style).
友だちには普通体で話してもいいよ。
You can speak in the plain style with friends.
文章を書くとき、学術論文では普通体（である調）が使われることが多い。
When writing, academic papers often use the plain style ('de aru' style).
普通体と丁寧体の使い分けは状況や相手によって変わる。
The choice between plain style and polite style changes depending on the situation and the person you're speaking to.