To accept some situation or condition ahead of time. For example, to opt-in to an email campaign means that you agree 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, the user did opt-in for the program, which may or may not be true. See
spam and
bacn.