An OTP seed is a secret code or value used as the foundation for generating one-time passwords (OTPs) in authentication systems. This seed, combined with algorithms and sometimes the current time or a counter, creates unique passwords that are valid for only one login session or transaction. The security of OTP-based authentication relies on keeping the seed confidential. If an attacker gains access to the OTP seed, they can potentially generate valid OTPs and bypass security measures, compromising user accounts or systems.