What is eccentric?

First of all you need to go read the dictionary entries under concentric and eccentric. Once you have these principles down you are on the right track.
Then check out my diagram.
eccentric
The red one on the left represents my Fisher Rig with an eccentric BB design. The blue one on the right represents a standard BB shell with an eccentric adaptor (yesterday I called it an eccentricizer or something but the actual name is much more a corruption than this - ekzentriker?). Notice that the red and black inner rings on the left are the same size as the blue outer ring on the right. This is a 68mm diameter standard BB dimension.
In the Rig, the whole BB assembly is off centre so that when you spin the assembly the BB moves. The red and black inner rings represent the maximum range of adjustment this can provide.
The blue and black inner rings on the right represent the axle of a 2-piece crank at the extreme possible adjustment range. In fact the adjustment is only a few millimetres. A proper eccentric BB can accommodate a change in gearing (more or fewer teeth on the rear hub sprocket) with the same chain length, while the adaptor device allows you to obtain proper chain tension for one gear choice only.

Anyway, hopefully it all makes a bit more sense now…