Lagrange points (L1,L2)
In the Earth-sun system, the Lagrange points (or equilibrium points) L1 and L2 are located on a line containing the Earth and sun with L1 between the Earth and sun and L2 beyond the Earth away from the sun. L1 and L2 are unstable equilibria such that a spacecraft in orbit at either point requires regular adjustments to remain at either point.
There are Lagrange points other than just L1 and L2. More information including diagrams is available from the WMAP Mission web site.