To purposefully accept some situation or condition ahead of time. For example, to opt-in to an e-mail campaign means that you want to receive periodic newsletters or information, which may include advertising from the publisher or third parties. An opt-in program implies that you can cancel the service, or "opt-out." A lot of spam is sent out under the guise that, at one point, you did opt-in for the program, which may or may not be true. See spam.