Final answer:
An opt-in database, in the context of e-mail marketing, is a list of subscribers who have given consent to receive marketing emails from a company.
Step-by-step explanation:
In reference to e-mail marketing, an opt-in database is a list of c. subscribers who have agreed to allow a company to send them e-mails with marketing messages. These individuals have explicitly given their consent, usually by signing up on a website or formally agreeing to receive communications from the company. This consent-based approach is crucial for building trust and maintaining a positive relationship with customers, as well as for complying with regulations like the General Data Protection Regulation (GDPR).