1. choose or select, declare a preference
opt-in/out of notifications
The verb 'to opt' means to make a choice from a range of possibilities. If you 'opt-in' it means to become a part of something such as a group, subscription, plan or organisation. To 'opt-out' therefore is the opposite, to chose not to participate in something. To 'opt for' something means to choose it. "I opted for the seafood rather than the pasta." You will hear this expression used often in formal contexts where a choice is required to be made. "An opt-out system for organ donation would lead to a huge increase in available organs for transplant."