A state in which an organism or a cellular system, as defined by its full set of functionality, ceases to exist (i.e., most of the functions are irreversibly lost).
In terms of humans, there is a strict definition of death, namely brain death:
Brain death is defined as the absence of higher brain function or brain-stem reflexes, and when other factors that could mimic brain death, such as drugs or low blood pressure, have been ruled out (1).
Brain dead patients can be artificially maintained by means of life support that sustain blood flow and breathing. Still, they are clinically dead (1).
References
(1): WebMD
