A boysenberry is what you get when you cross a raspberry, a blackberry, and a couple of other berries. It's a soft, slightly tart fruit that's especially delicious in pies.
A California horticulturalist, Rudolph Boysen, developed the boysenberry in the 1920s. Boysen experimented with cross-pollinating raspberries, blackberries, and loganberries (hybrid berries themselves) in the hopes of cultivating a big, soft berry, but he eventually abandoned the project. Berry farmer Walter Knott rescued his few last plants several years later, named them after Boysen, and found commercial success selling the large, sweet-tart berries from his farm stand.