We introduce the possibility that a significant component of the energy
density of the universe has an equation-of-state different from that of matter,
radiation or cosmological constant ($\Lambda$). An example is a cosmic scalar
field evolving in a potential, but our treatment is more general. Including
this component alters cosmic evolution in a way that fits current observations
well. Unlike $\Lambda$, it evolves dynamically and develops fluctuations,
leaving a distinctive imprint on the microwave background anisotropy and
mass power spectrum.