A domain hack is an unconventional domain name that combines domain labels, especially the top-level domain (TLD), to spell out the full "name" or title of the domain, making a kind of pun.
For example the second-level domain (SLD) blo.gs makes use of the TLD .gs (South Georgia and the South Sandwich Islands) to spell "blogs". The third-level domains del.icio.us and cr.yp.to make use of the SLDs icio.us and yp.to from the TLDs .us (United States) and .to (Tonga) to spell "delicious" and "crypto" respectively.