Otona (大人) means "adult." It refers to a mature person, typically someone who has reached full physical and mental development, or legal age. It is the antonym of "kodomo" (child) and is used when distinguishing between adults and children. Example: 彼はもう大人です。(Kare wa mō otona desu.) = He is already an adult.

成熟(せいじゅく)した人間(にんげん)を指(さ)す言葉(ことば)で、「子供(こども)」の反対(はんたい)です。例(れい)：彼(かれ)はもう大人(おとな)です。