Adipose fin is a fin that salmonids (salmon, trout, char, whitefish) have. It is located on the back between the dorsal fin and the tail. It is fleshy, soft, and its purpose is not known. It is a good characteristic for differentiating salmonids and minnows.
Salmonids that are raised at hatcheries usually have their adipose fin clipped prior to their release into rivers and lakes. These are known as hatchery-marked fish.