The fan-owned south coast club said on their website (www.portsmouthfc.co.uk) that the agreement would allow a 70-day period of negotiations "regarding a possible purchase of the club".
"Mr Eisner and Tornante will be reviewing our business and financial information, and equally we will be checking out their own respective financial and business credentials," they added.Portsmouth were league champions in the 1948-49 and 1949-50 seasons, and won the FA Cup in 1939 and 2008 before going into administration in 2010 and suffering three relegations.
Currently chasing promotion, Pompey came out of administration in April 2013 with a supporters' trust taking over.
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Portsmouth said on Thursday that the board and Eisner "will look to negotiate the terms of what would effectively be the best and final offer they wish to make for the club".
Trevor Birch, a former administrator of the club, has been retained by the board in an advisory capacity and to liaise with stakeholders.
Eisner, 75, left Disney in 2005 and is also a former president of Paramount Pictures. The Tornante Group is his media investment vehicle.
The Pompey Supporters Trust said it had a duty to listen to any serious or credible offer for the club and the shareholders would decide the outcome.
"Retaining an ownership stake will be central to any discussions we enter into," it said this week when reports of Eisner's interest emerged.
(Reporting by Alan Baldwin, editing by Ed Osmond)
