Rolling without slipping কী?

Two solids $S_1$ and $S_2$ touch at a point $I$. The slip velocity of $S_1$ on $S_2$ is $\vect v_{\text{slip}} = \vect v_{I \in S_1} - \vect v_{I \in S_2}$, the difference of the velocities of the two material points that coincide at $I$ at this instant. $S_1$ rolls without slipping on $S_2$ when $\vect v_{\text{slip}} = \vect 0$. For a wheel of radius $R$ rolling without slipping on a fixed ground, with centre velocity $v_G$ and angular velocity $\omega$, the condition reads $v_G = R\omega$ (signs chosen so that a wheel rolling to the right turns clockwise).