I agree it's the same as "opt-out" but I like the phrase "opt-in by default" because it implies there's an affirmative "I want to participate in this" option so

I agree it's the same as "opt-out" but I like the phrase "opt-in by default" because it implies there's an affirmative "I want to participate in this" option somewhere, and that it is set to "yes" by default. IMO it properly reflects that what looks like an active affirmative choice by the user is actually not.