Immortality is the ability to live forever, or put another way, it is an immunity from death. It is unknown whether human physical (material) immortality is an achievable condition —biological forms have inherent limitations which may or may not be able to be overcome through medical interventions or engineering. And even should human biological immortality be achieved, people could still continue to die from unforeseeable traumatic events.
In religious (typically Christian) contexts, immortality is often stated to be among the promises by God (or other deities) to human beings who show goodness or else follow divine law (cf. resurrection). Moreover, only God is regarded as truly immortal, hence it is only through God's resources for resurrection and salvation that human beings may transcend death and live eternally.