I've seen a few posts where it was used to describe ordering something from the children's menu and having it upsized to a larger portion and using an adult's dining credit to pay for it. I think it's an informal term because there isn't a junior menu at any restaurant in DW. It definitely doesn't exist on the dining plan where there are only 2 age categories: ages 10 years old and up and child 3-9.
For resort reservations, however, there is a junior classification. It is someone between the ages of 10 and 17 who has to purchase an adult park ticket, but is free in terms of room occupancy. Once a person turns 18, however, if that person is the third adult in the room, the additional adult fee is charged.