In Italian, what is the meaning of 'redente' and what is its historical meaning?

It is the plural of redento.
As an adjective, it means redeemed. As a verb, it is the past participle of redimere.

http://en.wiktionary.org/wiki/redente (showing that redente is a plural of redento

http://en.wiktionary.org/wiki/redento (showing the meaning of the word as an adjective and as a verb)

The plural past participle of "redimere" (to redeem).

http://en.wiktionary.org/wiki/redimere#Italian

http://en.wiktionary.org/wiki/redento

Redeemed territories?

You can often read "nella terra redente" or "sulle terre liberate e redente" referring to the era after fascism, meaning "in or on this liberated and redeemed land".

http://www.unilibro.it/find_buy/Scheda/libreria/autore-non_specificato/sku-908219/commissione_parlamentare_d_inchiesta_sulle_terre_liberate_e_redente_.htm

http://www.antiqbook.com/boox/natura/3771_dom.shtml

http://www.fig.net/pub/fig2006/papers/ts60/ts60_01_lun_0208.pdf

I think the answer qualifies.

Quote:
Italian irredentism (Italian: Italia irredenta) was an Italian nationalist Irredentist movement that aimed to complete the unification of all ethnically Italian peoples. Originally, the movement promoted the annexation by Italy of territories inhabited by an Italian majority but retained by the Austrian Empire after 1866 (hence 'unredeemed' Italy). These included the Province of Trento, and Trieste, but also areas with a South Slavic ethnic majority, such as Istria, Dalmatia, and the city of Rijeka. The ideology was then extended to Corsica, Gorizia and Gradisca, the Ionian islands, Malta, Nice, and Ticino.

Not a formal organization, it was rather an opinion movement that claimed that Italy had to reach its 'natural borders'. Similar patriotic and nationalistic ideas were common in Europe in the 19th century.

https://en.wikipedia.org/wiki/Italia_irredenta