---
title: "Definition of life"
chunk: 1/3
source: "https://en.wikipedia.org/wiki/Definition_of_life"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:17:39.577696+00:00"
instance: "kb-cron"
---

The precise definition of life is a contested aspect of it, and several proposals have been advanced. Biology defines and studies life as we know it, but abiogenesis and astrobiology seek wider and more encompassing definitions. Abiogenesis is the process by which life develops from inorganic materials, so a definition aims to establish the frontier between inorganic matter and the earliest lifeforms. Astrobiology seeks extraterrestrial life, which may differ from life on Earth.

== Common features ==

Life does not have a simple definition, because life on Earth has a huge diversity, ranging from microscopic microorganisms to massive plants and trees, and in all sorts of habitats. A common way to define life is by using a number of characteristics that should be common to all life forms. However, those characteristics are not universal, and there are exceptions and possible false positives with all of them. The main ones include:

Order: The elements that make up life are not randomly distributed. There is a biological organisation at all levels of life, from the microscopic cell to a full organism and even to the groups of several organisms. However, order is a necessary but not sufficient condition for life, as other structures like crystal rocks are also capable of displaying order. At the atomic level, atoms and molecules have order, but they are not considered to be alive, only to be the building blocks of life.
Reproduction: Living organisms are capable of reproduction, a process where an individual organism can create a new, independent organism that is almost similar to the original. Cells reproduce by splitting, larger organisms may use more complex processes such as sexual reproduction. Hybrids like mules and individuals incapable of reproduction as a result of infertility or other diseases are not considered exceptions, as they were born by the reproduction of other individuals. Reproduction may likely be a universal trait of life, as it allows life to persist despite the eventual death of individuals.
Growth: All living organism grow and develop during part or all of their life cycle. This follows patterns directed at least in part by heredity. However, growth alone is not enough to define life, as fire can grow but is not alive.
Homeostasis: All Earth lifeforms use energy for their internal work, and it is unlikely for any potential extraterrestrial life to be different about that. From a physical point of view, a lifeform is a thermodynamic system, and the second law of thermodynamics means that if left alone it would gradually lose its internal energy, leading to increased disorder within its components, causing its biological death. To avoid that, the lifeform requires a frequent source of energy. Usually it is food, matter that is transformed by the lifeform's internal biology into usable energy, in line with the first law of thermodynamics. In turn, the environment needs an external source of energy (such as sunlight or the planet's internal heat) so that the lifeforms do not deplete the free energy. It is unlikely that life can exist long-term without such a source of energy.
Interaction with the environment: All lifeforms react in some way to certain stimulus, changes in the environment. Plants may grow in the direction of sunlight as a result of phototropism to aid their photosynthesis, and animals may leave cold areas for warmer ones to keep their internal heat within acceptable levels.
Evolution: Living organisms interact with their environment, and with other organisms, both from their same species and from others. Evolution is a process that makes new organisms, generated by reproduction, to be better suited for those interactions.
Trivial definitions of life, such as those used in dictionaries and science divulgation, rely on several aspects that should take place in it, such as homeostasis, growth, reproduction, and death. Biology, however, provides a more reliable answer: all lifeforms on Earth are composed of cells (both unicellular and multicellular lifeforms), and reproduction replicates information from an ancestor into its offspring with the work of the DNA and the RNA. All lifeforms on Earth have this in common, and nothing that does not live does. It is, thus, a perfect working definition for most sciences. However, it is an incomplete definition for abiogenesis, the science that studies the origin of life. Earth began completely lifeless, and by some unclear chemistry inorganic materials combined themselves and created life. But life as we know it is too complex to appear abruptly, the process must have had steps, and we would require a better definition of life to decide which of those steps can be considered lifeforms, even if more primitive. As for astrobiology, all lifeforms known to us are from a single planet. Life in other planets may have developed in other ways, and we would need a broader definition that would cover such divergent lifeforms as well.

== Problems ==