A domain hack is an unconventional domain name that uses parts other than the SLD (second level domain) or third level domain to create the full title of the domain name. Popular websites featuring a domain hack are del.icio.us, blo.gs, rome.ro, tcl.tk, inter.net, dau.pl, and cr.yp.to.

Typical domain names follow the structure of "example.com", where "example" is the SLD and "com" is the TLD (top level domain). A domain hack could be "examp.le", where "examp" is the SLD and "le" is the TLD (however this is an impossible example as "le" is not an existing TLD). A domain hack can be composed of more than just a domain name, and also make use of the server's file structure. Various structures such as "examp.le", "exam.ple", "ex.am.ple", "ex.am/ple", are all possible structures used to create domain hacks.

The most popular and most sought after domain hacks are those that use only the SLD and TLD (top level domain) to construct the full title, as shown by the "examp.le" example structure, above. However, a domain hack may use third level domains, fourth level domains, etc., and even directories/folders after the domain name to construct the title. A non-working, extreme, example of this would be http://do.ma.in/ha/ck/. More domain hacks are possible than one might assume due to the approximately 300 gTLDs and ccTLDs that exist all around the world.

Some TLDs only permit registrations at the third or higher level, beneath names that represent some grouping of sites; however, in some cases, these names spell words themselves, such as co.in (coin) and co.il (coil)

hmm is it really a hack? I think of a hack as compromising security. I think its more a play on words