A pulsar is a neutron star which emits beams of radiation that sweep through the earth's line of sight. Like a black hole, it is an endpoint to stellar evolution. The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis. Pulsars pulse because the rotation of the neutron star causes the radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period. External viewers see pulses of radiation whenever this region above the the magnetic pole is visible. Because of the rotation of the pulsar, the pulses thus appear much as a distant observer sees a lighthouse appear to blink as its beam rotates. The pulses come at the same rate as the rotation of the neutron star, and, thus, appear periodic.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 188.8.131.52.0