  We consider a general class of intermittent maps designed to be weakly
chaotic, i.e., for which the separation of trajectories of nearby initial
conditions is weaker than exponential. We show that all its spatio and temporal
properties, hitherto regarded independently in the literature, can be
represented by a single characteristic function $\phi$. A universal criterion
for the choice of $\phi$ is obtained within the Feigenbaum's
renormalization-group approach. We find a general expression for the dispersion
rate $\zeta(t)$ of initially nearby trajectories and we show that the
instability scenario for weakly chaotic systems is more general than that
originally proposed by Gaspard and Wang [Proc. Natl. Acad. Sci. USA {\bf 85},
4591 (1988)]. We also consider a spatially extended version of such class of
maps, which leads to anomalous diffusion, and we show that the mean squared
displacement satisfies $\sigma^{2}(t)\sim\zeta(t)$. To illustrate our results,
some examples are discussed in detail.
