Death
Death is the permanent termination and cessation of conscious activity within any form of matter, whether through the shut down of the body system, caused by enough damage and fatal disorganization. Death is commonly considered a sad or unpleasant concept, due to the termination of social bonds with the deceased. The fear of death is extremely common in sapient beings. Because of that fact, religion was created as a coping mechanism.
Because of the paradoxical and incomprehensible nature of death, many religions have their own theories and assumptions of what happens after one dies. A common belief would be that of the afterlife where the living being's soul and/or spirit goes to, whilst others may believe you simply ceased to exist or become a ghost. In Christianity, for example, God uses this fear of death to manipulate and gain further control over people by promising his followers that there is a heaven for the righteous and the faithful, while there is a hell for anyone who doesn't believe in him and the ones who oppose him. Rituals for the deceased vary from one culture to another, with the most common being a funeral which is found throughout cultures and allows the friends and family of the deceased to express and share their grief with one another and to bid farewell and even a safe journey to the preferred afterlife of the deceased.
Any form of matter with consciousness is able to die. This includes immortals, gods, and sea stars/starfishes who all have the ability to regenerate but if their entire cellular or spiritual composition is destroyed, they will die.