From a single file line of two: Leader does a Quarter right (Zig) and the trailer does a Quarter Left (Zag).
NOTE: These terms are often used as an addition to Tag the Line (ie., Tag the Line "Zig Zag").
Bill Shymkus and Jack Lasry 1968
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.