Multi-device access is the ability to use one subscription or account on different screens or hardware, such as a phone, tablet, smart TV, or browser. In streaming and other online services, it depends on account entitlement, login sessions, and device recognition so the same user can switch devices without losing access.
It matters in cyber security because every extra device adds more account handling and more opportunities for abuse. Attackers may try credential theft, account sharing beyond policy, or token hijacking to use a paid account on multiple endpoints. Defenders reduce risk with device limits, session timeouts, step-up authentication, reauthorization prompts, and the ability to revoke unknown devices. Good multi-device design balances convenience with control, so legitimate switching stays smooth while unauthorized access is detected quickly.