The term "buttery" is used to refer to smooth background (like melted butter) as opposed to jagged background. TCAV already provided a link showing the donut affect. It should also be noted that how the 'bokeh' looks is affected by what is in the background. It's easier to end up with a smooth looking bokeh when the background is uniform. Like this:
Same camera and lens - not as tightly framed (so greater DOF) combined with leaves in the background which don't blur as nicely and you get:
Not bad, but not as pleasing. My point in posting both is that bokeh isn't simply a matter of the lens. It's a matter of DOF, distance from your subject to background and what that background is.