Identity reuse is the practice of using the same identifier, such as an email address, username, phone number, or device token, across multiple services. In security and privacy terms, that shared identifier becomes a linkable trail. Once one site knows the identifier, other systems, data brokers, or attackers may use it to correlate accounts, build profiles, or guess where the same person is active online.
It matters because reused identities increase the impact of a single leak or phishing compromise. If one account is exposed, attackers can target other services with password resets, credential stuffing, or social engineering. Defenders reduce this risk by encouraging unique identifiers, aliases, and strong account separation. Disposable email aliases are one practical control: they keep services from sharing a stable inbox and make cross-service tracking harder. The protection is useful, but it does not stop tracking through other metadata such as payment details, phone numbers, or browser signals.