An evening bag is designed for formal evenings, dinners, events, and occasions where you carry minimal essentials — phone, cards, a key, lipstick. Size is typically small or very small; materials are formal (satin, velvet, metallic leather, beaded, crystal-embellished). The bag is an accessory in the same sense as jewellery: it completes an outfit rather than serving a primary functional role.
The distinction between an evening bag and a clutch is almost semantic. Most clutches are evening bags; some evening bags have a short chain or strap. Judith Leiber's jewelled minaudières represent one extreme of the category. Chanel's WOC (Wallet on Chain) — a chain-strap flat case — is the most collected contemporary evening bag precisely because it works both for evenings and, at a stretch, for day use.