Chris Langton, whom many regard as the founder of the research discipline known as Artificial Life, defines the subject as the study
``of man-made systems that exhibit behaviors characteristic of natural living systems. It complements the traditional biological sciences concerned with the analysis of living organisms by attempting to synthesize life-like behaviors within computers and other artificial media. By extending the empirical foundation upon which biology is based beyond the cabon-chain [sic] life that has evolved on Earth, Artificial Life can contribute to theoretical biology by locating life-as-we-know-it within the larger picture of life-as-it-could-be.'' [Langton 88] (p.1, original emphasis).
As a brief aside, one might argue that the implication in this quotation that traditional theoretical biologists are concerned only with terrestrial biology is not entirely accurate. Although the approach may be different, the goal of attempting to develop a truly universal theory of biology (i.e. a general theory which might, for example, predict the forms life might take on other planets, in contrast to our rather provincial view of life on Earth) is one that is shared by many biologists--for example, John Maynard Smith (e.g. [Maynard Smith 86] pp.21-23) and Richard Dawkins (e.g. [Dawkins 83]), to name but two.
Langton's publications in the mid-1980s in which he brought the term `artificial life' into common usage (e.g. [Langton 86]),3.2 together with his organisation of a workshop on the subject in Los Alamos in September 1987 (which has subsequently grown into the biennial International Conference on Artificial Life), were certainly the principal factors contributing to the current popularity of the subject (and to its crystallisation as a coherent subject3.3). However, research which would today certainly be classified as `artificial life' began almost as soon as the modern digital computer was developed, with Nils Barricelli's work at the U.S. Institute for Advanced Study commencing in 1953, described in Section 3.2.2. The theoretical background for the subject was being developed even earlier, by von Neumann [von Neumann 66], for example, and by proponents of the cybernetics field (e.g. [Weiner 48]). A good history of much (but certainly not all) of this earlier work is provided by Langton himself in [Langton 88].