The question of internet domains and extensions is not the only source of confusion here. There is also the problem that even if you manage to concentrate your searches on matters of corruption, you find both "extensión de dominio" and "extinción de dominio" apparently referring to the same thing. They seem to be used interchangeably; there are a whole series of web pages that fluctuate apparently arbitrarily between the two. Here's one example, about Bolivia:

"DELGADO ANUNCIA REFORMAS AL PROYECTO DE LEY DE EXTINCIÓN DE DOMINIO DE BIENES
La presidenta de la Cámara de Diputados, Rebeca Delgado, afirmó este viernes que existen “tres aspectos” de la Ley de Extensión de Dominio que podrían ser modificados en un 60 por ciento."
http://anbolivia.blogspot.com.es/2012_10_21_archive.html

As I say, I've found this happening again and again. Here a reference to "extensión de dominio" even slips into a Mexican parliamentary document on "extinción de dominio" (see p. 2):
http://www.diputados.gob.mx/cedia/sia/spi/SAPI-ISS-59-12.pdf

But as someone protests in this forum, "extensión de dominio" seems to be an error, and the correct term is "extinción de dominio":

"Espero que sea un craso error de quien escribio la nota y que no sea otra ineptitud de los legisladores del partido que sea...la ley a la que se quieren referir y que ya existe en el ambito federal es LEY DE EXTINCIÓN DE DOMINIO... no de EXTENSIÓN de dominio..."
http://www.urbanfreak.net/showthread.php/5879-Guillermo-Padr...

And this is quite true; there is undoubtedly a Ley de Extinción de Dominio in Mexico, Colombia and a number of other Latin American countries, but no reliable evidence of a Ley de Extensión de Dominio anywhere. Here's Mexico's:
http://www.diputados.gob.mx/LeyesBiblio/pdf/LFED.pdf

The confusion, which is quite widespread, must be based on the fact that "extinción" and "extensión" sound very similar in American Spanish.

Anyway, "dominio" means ownership, and "extinction of ownership", as it's translated in some places, is cancelling or annuling ownership or property rights in illegally acquired assets. The legal term for this in English is in rem forfeiture:

"UNODC
United Nations Office on Drugs and Crime
Model Law on In Rem Forfeiture
[...]
It is a regional initiative because it was designed following the civil tradition of those Spanish-speaking countries in Latin America that could incorporate the model in its domestic law. The term extinción de dominio ” (in rem forfeiture) was adopted because it was the most commonly used term in the region rather than, for example, decomiso sin condena (non-conviction based forfeiture), a term used in other international areas."
https://www.unodc.org/documents/colombia/2013/septiembre/MOD...

On forfeiture, see here:

"Under the 1978 amendments, the federal government was authorized to proceed in rem against property. In rem forfeiture actions are taken against the property itself, not against its owner. In such proceedings, the guilt or innocence of the property owner regarding any criminal activity is irrelevant. Thus, under the Forfeiture Act, the government may remove property from persons it suspects of a crime, without ever charging them with a crime. The basis of this kind of forfeiture is traced back to the deodand doctrine of the English common law."
http://legal-dictionary.thefreedictionary.com/forfeiture

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Note added at 2 hrs (2014-02-17 01:59:00 GMT)
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If you agree that "extensión" is an error for "extinción", translator's note seems to be called for. But honestly, "extensión de dominio" really doesn't seem to make any sense.