International Encyclopedia of Systems and Cybernetics

1) The global set of individuals or elements of a same class, i.e. sharing defined characteristics.

A population is generally confined within a limited space, possibly with very different local densities.

While belonging to the same class, no individual is absolutely identical with any other: populations are generally polymorphic, within limits.

A population is not only synchronic, but also diachronic since the number of members may increase or decrease with time, as well as their repartition and global or local densities.

Populations correspond to a special type of systems, in which every element is directly and dominantly interconnected with the environment and only loosely and intermittently with other members of the population (see "Scattered system"). This type of systems does not generally possess clearly identifiable sub-systems.

2)"An aggregate of individuals conforming to a common definition, to which individuals are added (born) and substracted (die) and in which the age of the individual is a relevant and identifiable variable" (K. BOULDING, 1956, p.13)

BOULDING explains: "Birth" occurs when an item begins to conform to the definition which encloses the aggregation, and "death" when the item ceases to conform to the definition. A definition may be thought of as closed fence: everything inside the fence belongs to the defined population; birth consists in crossing the fence into the enclosure; death in crossing the fence out of the enclosure. The population concept thus defined is a perfectly general one, and applies not only to human or animal populations, but to populations of automobiles, poems, stars, dollars, ideas, or anything that is capable of definition" (1956, p.67).

In any system that can be described as a population, internal and external interactions can be considered in terms of competition, complementarity, parasitism or symbiosis whether the species consist of animals, commodities, social classes or molecules" (Ibid, p.13).

3) A generalized and more abstract definition of population is given by T.A. and W. MILLINGTON: "The whole set of items which have a common characteristic, which is subject of some sampling in the process of statistical analysis" 1971, p.182).

Along this line, populations can be sampled, i.e. an appropriate (and frequently numerically quite limited) subset can be studied for some specific characteristics, in view to obtain generalizations by the use of convenient statistical methods.

Samples are however referred to a specific moment in time. As a result, any dynamic inquiry needs various successive samplings to study temporal change affecting the population.