What is an adipose fin and why name a simple website about trout after such a thing?

First off, adipose means 'fatty tissue'. Not exactly glamorous, but the adipose fin is somewhat unique in the world of fish; most don't have one. Salmonids do. Every single species of trout, salmon, whitefish, grayling, char, lenok and huchen have a small, extra fin on their back between the dorsal fin and the caudal fin (tail).

In nature, the adipose fin does exactly nothing. It's entirely vestigial. But nature aside, the adipose fin has been assigned a purpose for the sake of fisheries management. In areas(generally anadromous waters) where hatchery trout or salmon are planted alongside a wild population, the adipose fin is removed from hatchery fish when they are fingerlings. This process allows later identification as wild or hatchery to be made easily by anglers - more specifically, it allows harvest to be limited to hatchery fish - in fact, in most cases harvest of hatchery-born steelhead and salmon is encouraged to lessen the genetic impact they could have if they were to successfully spawn and intermingle with the wild stocks. Wild fish, with adipose fins intact are to be released, alive, back into the water to continue their journey to their natal streams to spawn.

So, Adipose Fin, a website about wild trout.