The K correction
  The K correction "corrects" for the fact that sources observed at different redshifts are, in general, compared with standards or each other at different rest-frame wavelengths. It is part of the relation between the emitted- or rest-frame absolute magnitude of a source in one broad photometric bandpass to the observed-frame apparent magnitude of the same source in another broad bandpass. This short pedagogical paper provides definitions of and equations for the K correction. 
David W. Hogg (NYU), Ivan K. Baldry (JHU), Michael R. Blanton (NYU),   Daniel J. Eisenstein (Steward)