Death was a word that referred to the opposite of life. When life was gone from someone, they were dead, or deceased, and this state was referred to as the state of "death," When the body no longer has its' soul.
Zombies and other like-creatures were referred to as Undead, as they were dead, but they had the appearance of being alive, except for the fact that they walked around in a mindless state. Ghosts were also by definition dead, at least their corporeal bodies were, as their spirit was still alive and active.
Sometimes, although the body died, the soul would go on living. This concept was known as the afterlife. It usually meant that when a person's body dies, his or her soul continued to exist, as it was immortal -- the body essentially being only the shell of a person.
Usually, a good soul would go to heaven, whereas a bad soul would go to hell, or another place of fiery torment in the Nether World. Still, sometimes souls wandered the Earth. These souls were called ghosts, although many other names were used depending on the type, such as phantom and wraiths.
In the comics
In DC Comics, death is seen far more than it is in the SFU. For the most part death is censored in Super Friends and other Saturday morning cartoons, but this isn't the case in the comics, and in many ways death (and resurrection) in comics has became a gimmick to sell comic books. The most famous death was Superman's death, in Superman Vol. 2 # 75 (January 1993). This death however, only proved to be quite temporary, as he was later brought back to life in a later issue. Flash was also killed during the Crisis on Infinite Earths, only to be brought back in Flash: Rebirth. Supergirl was also killed during the Crisis but because of the fact that she was the Earth-One Supergirl, her death had no affect on the New Earth Supergirl, which was introduced in Superman/Batman #8 (May 2004). Hal Jordan also had died, and he became the new Spectre.