What is Tacticity?

In a vinyl polymer $\ce{-[CH2-CHR]_n-}$, every $\ce{CHR}$ carbon is a stereocentre. The tacticity describes their relative configurations along the chain drawn as a planar zig-zag: in an isotactic chain all the $\ce{R}$ groups lie on the same side of the plane, in a syndiotactic chain they alternate, and in an atactic chain they are placed at random.