Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from matter that does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy transformation, and reproduction.[2][3] Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. Biology is the science that studies life.

The gene is the unit of heredity, whereas the cell is the structural and functional unit of life.[4][5] There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many biomolecules such as proteins and nucleic acids. Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells and passes its genes onto a new generation, sometimes producing genetic variation.

Organisms, or the individual entities of life, are generally thought to be open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve over multiple generations. Other definitions sometimes include non-cellular life forms such as viruses and viroids, but they are usually excluded because they do not function on their own; rather, they exploit the biological processes of hosts.[6][7]

Abiogenesis, also known as the origin of life, is the natural process of life arising from non-living matter, such as simple organic compounds. Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale, but it has also adapted to survive in most ecosystems and conditions. New lifeforms have evolved from common ancestors through hereditary variation and natural selection, and today, estimates of the number of distinct species range anywhere from 3 million to over 100 million.[3][8]

Death is the permanent termination of all biological processes which sustain an organism, and as such, is the end of its life. Extinction is the term describing the dying-out of a group or taxon, usually a species. Once extinct, the extinct species or taxon cannot come back to life. Fossils are the preserved remains or traces of organisms.

Definitions
The definition of life has long been a challenge for scientists and philosophers.[9][10][11] This is partially because life is a process, not a substance.[12][13][14] This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside of Earth.[15][16] Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living.[17] Legal definitions of life have also been described and debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.[18] As many as 123 definitions of life have been compiled.[19]

Biology
See also: Organism
Since there is no consensus for a definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment. This characteristic exhibits all or most of the following traits:[11][20][21][2][22][23][24]

Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
Organisation: being structurally composed of one or more cells – the basic units of life
Metabolism: transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organisation (homeostasis) and to produce the other phenomena associated with life.
Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
Adaptation: the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.[25][26][27]
Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.
These complex processes, called physiological functions, have underlying physical and chemical bases, as well as signaling and control mechanisms that are essential to maintaining life.