To clear up a possible source of confusion, I mention that the critical density has often been described in the semi-popular literature as the borderline between eternal expansion and eventual collapse. If Einstein's cosmological constant is zero, as most of us thought a few years ago, then this definition is equivalent to the one given above. Recent evidence, however, suggests that the cosmological constant may be nonzero, in which case the two definitions are not equivalent. In that case, the one given in the text agrees with the definition used in the technical literature, and is also the definition that is relevant to the current discussion.