This is the 432nd most frequent Japanese word.
大人
"Adult" or "mature person."
The word '大人' refers to an adult or mature person who behaves responsibly.
彼は大人のように振る舞いました。
He behaved like an adult.
The word '大人' is used here to indicate the adult demographic as opposed to children.
この本は大人にも子供にも人気があります。
This book is popular with both adults and children.
In this sentence, '大人' is used in the context of pricing categories, referring to individuals above a certain age.