For the fun of it, here are a few examples of definitions:
To understand complex biological systems requires the integration of experimental and computational research -- in other words a systems biology approach. (Kitano, 2002)
Systems biology studies biological systems by systematically perturbing them (biologically, genetically, or chemically); monitoring the gene, protein, and informational pathway responses; integrating these data; and ultimately, formulating mathematical models that describe the structure of the system and its response to individual perturbations. (Ideker et al, 2001)
[...]the objective of systems biology [can be] defined as the understanding of network behavior, and in particular their dynamic aspects, which requires the utilization of methematical modeling tightly linked to experiment. (Cassman, 2005)
By discovering how function arises in dynamic interactions, systems biology addresses the missing links between molecules and physiology. Top-down systems biology identifies molecular interaction networks on the basis of correlated molecular behavior observed in genome-wide "omics" studies. Bottom-up systems biology examines the mechanisms through which functional properties arise in the interactions of known components. (Bruggeman and Westerhoff, 2007)
Why is it so difficult to come up with a concise definition of systems biology? One of the reasons might be that every definition has to respect a delicate balance between "the yin and the yang" of the discipline: the integration of experimental and computational approaches; the balance between genome-wide systematical approaches and smaller-scale quantitative studies; top-down versus bottom-up strategies to solve systems architecture and functional properties. But despite the diversity in opinions and views, there might be two main aspects that are conserved across these definitions: a) a system-level approach attempts to consider all the components of a system; b) the properties and interactions of the components are linked with functions performed by the intact system via a computational model. This may in fact reveal another source of difficulty when trying to define systems biology, which is to find a general and objective definition of "biological function" (or Lander's "goal of the system", see our brief post Teleology and Systems Biology). Feel free to comment and suggest on this...
Posted originally by Thomas on July 23, 2007