A somewhat formal noun used to refer to the full details or particulars of a matter. Often appears in set phrases like 委細を説明する or 委細承知.
事件の委細を上司に報告した。
I reported the details of the incident to my boss.
委細は追ってお知らせします。
Further details will be provided later.
Compound of 委 (entrust, detail) and 細 (fine, detailed). The exact historical derivation is uncertain, but the combination conveys 'detailed matters'.