A fictitious force (also called a pseudo force) is an apparent force that acts on all masses in a non-inertial frame of reference, such as a rotating reference frame. So, a fictitious force is one that seems to exist because a frame of reference is accelerated. The forces may be treated algebraically like a real force, but the fictitious forces do not necessarily obey Newton’s first law.

One example is the centrifugal force, another one is the Coriolis force.

Wikipedia entry