I'd probably refer to "footwork" as "ashi sabaki" and probably say that "tai sabaki" more means "body movement." Both, to me, are general terms referring to a group of movements like "tenkan," "irimi," "tenshin," "tenkai," and so on.
More specifically, I have heard the "tenkan" footwork being described as pivoting on one foot while the other foot sweeps behind -- ie if two are done successively, it's kind of like drawing a circle/ellipse on the mat with one leg while the other stays in the center.
-- Jun
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