An allograft is a graft using tissue that did not originate from the patient but from another person.
An allograft is in reality a 'transplant' - where a graft is 'harvested' (taken) from one person ('the donor') and put into another person ('the recipient'). In knee surgery, we usually refer to allografts in the context of the menisci or cruciate ligament. In the latter, hamstrings tendon, fascia lata, patellar tendon or achilles tendon may be taken taken from the donor and freezedried to eventually replace a damaged cruciate ligament in the recipient.
(NB. The word is never spelled or pronounced 'allograph' - this is a typo that is often found in consumer bulletin boards.)
An 'allograft' is a graft of material from the body of one person (usually a dead donor) to that of another person (the living patient), in which it takes hold, develops a new blood supply and grows.