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Definitions
There are many different definitions of aging, which don't necessarily fall into the broad idea of "getting older". There are a number of kinds of aging such as: biological, individual and population.

Defining aging is a difficult task; there are many different ways to define what aging encompasses. Terms such as mortality can be used to define biological aging, which refers to an organism’s increased rate of death as it progresses throughout its lifecycle and increases its chronological age. Another possible way to define aging is through functional definitions, of which there are two main types. The first describes how varying types of deteriorative changes that accumulate in the life of a post-maturation organism can leave it vulnerable, leading to a decreased ability of the organism to survive. The second is a senescence-based definition; this describes age-related changes in an organism that increase its mortality rate over time by negatively affecting its vitality and functional performance. An important distinction to make is that biological aging is not the same thing as the accumulation of diseases related to old age; disease is a blanket term used to describe a process within an organism that causes a decrease in its functional ability. Biological aging is defined as a continuum.

Aging is a common occurrence among most living things and is a permanent gradual physiological cellular decay. Various theories of aging include telomere shortening, generation of free radicals, accumulation of mutations, all of which contribute to this decay.

Antagonistic Pleiotropy is comprised of mechanisms/traits that are advantageous during childhood and reproductive years but become harmful as we age. These mechanisms can promote aging. For example, oxidative stress induced by reactive oxygen species which causes oxidation of lipids, proteins, and other compounds.

Genetic predisposition coupled with environmental factors, such as the methylation of DNA which causes mutations in gene expression patterns. Developmental outcomes can affect the rate of aging, however, aging and development do not refer to the same processes. Different environmental stresses and influences can change developmental path of an organism it also changes how the organism ages on an individual scale.

Development is defined as the stage of life when functional change is positive. Functional change during development is characterized by growth and physical maturation. This stage includes childhood all the way to puberty, where individuals reach reproductive status. The developmental theory of aging poses that aging is a programmed molecular mechanism that is essential for embryonic maturation that eventually results in aging phenotypes.

Aging is the progression in time of cell maturation in organisms as it interacts with its surroundings. Cellular changes are induced by random changes in function and structure of cells and their components. These changes over time can lead to death. Taking into consideration of all the above information, this is as close as one could get to a broad definition of aging.