The concept of organism is an old one dating back to 1774 (or so claims In the almost 90 years since Webster 1913, the definition has changed somewhat and focuses upon organs -
  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
(above from

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:

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

The avid reader will bring up the "well, isn't fire alive then?" question which leads to the admittance that those items listed above, while many living things possess some (some don't possess all) and some non-living things (such as fire) possess all, this is not a good criteria for deciding what is an organism. This brings us to a new list for determining what is a living organism: Some scientists (such as Richard Dawkins) use only the last two criteria listed above to define what an organism is - thus including viruses (there is a bit of a debate as to the living status of viruses). One would assume this applies to physical (possibly robots though this is not yet an issue) and biological constructs rather than such nebulous things as ideas.