  The electron density, its gradient, and the Kohn-Sham orbital kinetic energy
density are the local ingredients of a meta-generalized gradient approximation
(meta-GGA). We construct a meta-GGA density functional for the
exchange-correlation energy that satisfies exact constraints without empirical
parameters. The exchange and correlation terms respect {\it two} paradigms:
one- or two-electron densities and slowly-varying densities, and so describe
both molecules and solids with high accuracy, as shown by extensive numerical
tests. This functional completes the third rung of ``Jacob's ladder'' of
approximations, above the local spin density and GGA rungs.
