Simultaneous streams is the maximum number of devices or playback sessions a single account can use at the same time. Streaming platforms use this limit as part of entitlement enforcement: when a user starts a stream, the service checks whether the account is allowed another active session.
In cyber security, this control matters because it helps reduce account sharing, detect credential theft, and limit abuse of paid access. Attackers who steal subscriptions often try to start many sessions at once, while defenders watch for unusual concurrency, device changes, or location shifts. Services may enforce the limit with session tokens, device fingerprints, IP checks, and automatic logouts. If these controls are weak, one compromised account can be shared widely; if they are too strict, legitimate users may be blocked or forced to reauthenticate. The goal is to balance usability with abuse prevention.