Affinity isolated peptide
A peptide or protein can be purified by affinity chromatography. The principle is that the peptide has some substrate to which it will bind, retarding its progress through the column compared to the rest of the peptide population. A couple of typical examples are:


Using a His6 tag put on recombinant molecules, IIRC it binds to Ni2+ trapped in the chromatographic resin and is eluted with a chelator like EDTA. 
Immunoaffinity chromatography, where you have a population of antibodies on the chromatographic resin which are specific to the protein that you want to purify (or alternatively having the immunogen bound to the resin so that you can purify the antibodies against it)


Mighty Wikipedia has a link about affinity chromatography here.