What term signifies an irreversible cessation of all vital functions?

The correct term that signifies an irreversible cessation of all vital functions is "death." In the context of biological and medical definitions, death refers to the point at which an organism's vital processes have completely and permanently ceased. This encompasses the failure of critical bodily functions, such as respiration and circulation, and signifies that the organism can no longer sustain any life processes.

Understanding this concept is fundamental in the field of mortuary science, as it establishes the basis for various practices and protocols following the loss of life.

The other terms, while related, do not specifically denote the complete and irreversible end of vital functions. "Termination" could refer to the ending of processes but does not specifically indicate biological functions. "Mortality" typically refers to the state of being subject to death or the incidence of death in a population, rather than the act or state of being dead itself. "Extinction" usually applies to species rather than individuals and refers to the point when a species no longer exists as a whole.