Delirium is a common syndrome complicating terminal illness. It is underrecognized partly because it is a difficult clinical concept. Consciousness, awareness, alertness, arousal, awakeness, vigilance, and attention are some of the terms used to describe the deficits occurring in delirium. Though interconnected, they are often loosely defined. Alertness is the primary impairment, and attentional deficits are objective clinical indices of the cognitive impairments of delirium. Simple bedside assessments of delirium are considered. The “deliriant” threshold and the symptomatic fluctuations of delirium are important concepts in the understanding of delirium. Jackson's conceptualization of the nervous system is relevant to delirium. Raising the deliriant threshold by multicomponent interventions is the intent of the palliative management of terminal delirium.