A cognitive bias where a third, less attractive option (the decoy) makes one of the other options seem much better.
Popcorn:
Small: \$3
Medium: \$6.50
Large: \$7
The Medium is the decoy. It makes the Large look like a great deal (only 50 cents more!). Without the Medium, you might buy the Small.
Marketing
Decoy Effect
The useless option that changes your mind