Formal term used in contracts, official documents, and legal contexts to indicate the date on which something expires or ends.
The contract's expiration date is the last day of next month.
Please check the visa's expiry date.
有効期限 is the common term for 'expiration date' on products, cards, etc., whereas 満了日 is more formal and used for contracts or official periods.
Compound of 満了 (manryō, 'expiration; completion of a term') and 日 (bi, 'day'). The reading び is a rendaku form of ひ.