Used in pricing contexts such as admission fees, transportation fares, and event tickets to indicate the adult rate, as opposed to child (小人・しょうにん) or intermediate (中人・ちゅうにん) categories.
See also: 小人 (しょうにん) · 中人 (ちゅうにん)
The adult ticket price at the movie theater is 1800 yen.
On this bus, the fare is 220 yen for adults and 110 yen for children.
小人 is the child fare/age category, while 大人 (だいにん) is the corresponding adult category in the same ticket or admission pricing system.
The reading だいにん is a Sino-Japanese on'yomi reading of 大人, used specifically in fee-related contexts. The more common reading おとな is a native Japanese word.