I'll settle for you since the girl I really like is unavailable.
Bob has a crush on Alice. He's been longing for her from afar for the longest time. When he finally works up the nerve to ask her out, he learns that Alice is already dating Charlie. Not wanting to be alone, Bob decides to start dating Carol rather than still be alone. Carol is the Consolation Love Interest. Her entire relationship with Bob is defined on the basis that Bob did not end up with Alice instead. If she learns that Bob "settled for her" she may develop into a Insecure Love Interest, that is, if she doesn't call him out on his Moral Dissonance. For extra irony, if she breaks up with Bob, expect him to suddenly develop feelings for Carol. Compare to Operation Jealousy, Romantic False Lead, and Romantic Runner-Up.
Examples:Anime Comic Books Literature Live-Action Television