thumb|250px|Fountain of Eternal Life, Cleveland, Ohio. It symbolizes "Man rising above death, reaching upward to his God and toward Peace."

Immortality (or eternal life) is the concept of living in a physical or spiritual form for an infinite or inconceivably vast length of time.

As immortality is the negation of mortality—not dying or not being subject to death—it has been a subject of fascination to mankind since at least the beginning of history. The Epic of Gilgamesh, one of the first literary works, dating back at least to the 22nd century BCE, is primarily a quest of a hero seeking to become immortal. What form an unending human life would take, or whether the soul exists and possesses immortality, has been a major point of focus of religion, as well as the subject of speculation, fantasy, and debate.

It is not known whether human physical immortality is an unachievable phenomenon or not. Biological forms have inherent limitations — for example, their fragility and slow adaptability to changing environments, which may or may not be able to be overcome through medical interventions, engineering, etc. On the other hand, biological immortality already exists among some simple, but multicellular life-forms.

Some scientists, futurists, and philosophers, such as Ray Kurzweil, believe that human immortality is achievable in the next few decades. Others are somewhere in the middle of these two extreme viewpoints, thinking that immortality is achievable in some period of time longer than 20–30 years, but not impossible. Aubrey de Grey, a researcher who has developed a series of biomedical rejuvenation strategies to reverse human aging (called SENS), believes that his proposed plan for ending aging may be implementable in two or three decades.

Biological immortality is what some life extension advocates feel is likely in decades to come. Specifically this refers to the absence of aging of the body due to baseline biological human limitations, but acknowledgement that complete immortality in a human form is unlikely due to the fact that even when you remain biologically young, once about every one thousand and seven hundred years individuals will get killed in an accident (4.4% of U.S. deaths now) or by other means .

Ultimately, a timeless existence is also not known for certain to be achievable, or even definable, despite millennia of arguments for eternity. Wittgenstein, in a notably non-theological interpretation of eternal life, writes in the Tractatus that, "If we take eternity to mean not infinite temporal duration but timelessness, then eternal life belongs to those who live in the present."

Spiritual

• The belief in an afterlife experienced by an immortal soul is a fundamental tenet of many branches of Christianity, Zoroastrianism, Hinduism, Islam, Judaism and the Bahá'í Faith.