A session cookie is a temporary cookie stored only in browser memory and deleted when the browser closes. It has no Expires or Max-Age attribute — that's what distinguishes it from a persistent cookie. Typical uses: shopping cart contents, login state during a visit, CSRF tokens, and multi-step form data. Session cookies are usually strictly necessary under GDPR and ePrivacy Article 5(3), so they're exempt from consent — but only when used purely for the user-requested function. A session cookie used for analytics or profiling loses the exemption and needs consent. Security best practice: pair with HttpOnly, Secure, and SameSite attributes.