1.
pending issue; unresolved matter; outstanding concern
A problem or issue that has remained unresolved over time and requires attention. Often used in formal, political, and business contexts for matters that have been put off or are proving difficult to settle.
Often used as a modifier: 懸案の問題 (pending issue), 懸案事項 (outstanding matter). Commonly modified by 長年の (long-standing) or 最大の (biggest). Frequently appears in news reports about diplomacy, politics, and organizational management. More formal than 課題 (task/challenge), with a stronger nuance of something that has been left unresolved.