Casual, informal speech used between peers; the opposite of polite honorific speech (keigo). It can be rude depending on the situation or the person.
先輩にタメ口で話すのは失礼だから、まずは敬語を使おう。
It's rude to speak casually to a senior, so use polite language first.
友達同士ならタメ口で話すことが多い。
Friends often speak to each other in casual language.
仕事の場では相手が望まない限りタメ口は避けたほうがいい。
At work, it's better to avoid casual speech unless the other person indicates it's okay.