What is difference between Nibbana and asaṅkhataṃ (unconditioned)?

Buddha says Nibbana is *ajātaṃ* *abhūtaṃ* *akataṃ* *asaṅkhataṃ*. Out of all these four, only *asaṅkhataṃ* (unconditioned) is something that I have to achieve. So can I say *asaṅkhataṃ* is Nibbana and vice versa? and by the way what is real meaning of *asaṅkhataṃ*? Buddha says its end of greed, aversion, and delusion... But I dont think ending of these three means ending of all past *sankhara*.. So, in the state of *asaṅkhataṃ* still lots of *sankhara* are there. It's just that they are infertile. So unconditioned is not right meaning of *asaṅkhataṃ* ... Ideally, it should be unconditionable? What do you say?