Polarization mode dispersion (PMD) is a form of modal dispersion where two different polarizations of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections and asymmetries, causing random spreading of optical pulses.
In this case, the fundamental mode has two orthogonal polarizations (orientations of the electric field) that travel at the same speed.
In a realistic fiber, however, there are random imperfections that break the circular symmetry, causing the two polarizations to propagate with different speeds.
Essentially, one splits the output of the fiber into two principal polarizations (usually those with dτ dω = 0, i.e. no first-order variation of time-delay with frequency), and applies a differential delay to re-synchronize them.
For example, a square or equilateral-triangle core would also have two equal-speed polarization solutions for the fundamental mode; such general shapes also arise in photonic-crystal fibers.