organism

1. a complex structure of interdependent and subordinate elements whose relations and properties are largely determined by their function in the whole
2. an individual constituted to carry on the activities of life by means of organs separate in function but mutually dependent : a living being

The criteria for "what is an organism" originally depended upon the concept of having organs - interdependent parts of the whole. Webster notes that simpler (such as unicellular organisms) do not have organs, but rather organelles - the cell itself being the smallest unit of a living thing something smaller than a cell cannot be living by itself (such as the mitochondria, chloroplasts, ribosomes, or other sub cellular structures).

Organisms are often defined as an entity that are alive:

• consumes nutrients
• produces wastes
• reproduction
• growth

Another definition of a living organism is (from Biology 3ed, Peter Raven and George Johnson, Mosby, New York, 1992):

• movement
• sensitivity
• death
• complexity

• Cellular organization
• Growth and metabolism
• Reproduction
• Passing of traits to offspring

Or"gan*ism (?), n. [Cf. F. organisme.]

1.

Organic structure; organization.

Grew.

2. Biol.

An organized being; a living body, either vegetable or animal, compozed of different organs or parts with functions which are separate, but mutually dependent, and essential to the life of the individual.

Some of the lower forms of life are so simple in structure as to be without organs, but are still called organisms, since they have different parts analogous in functions to the organs of higher plants and animals.

© Webster 1913.