Science (from the Latin scientia, meaning "knowledge" or "knowing") is the effort to discover, and increase human understanding of how the physical world works. Using controlled methods, scientists collect data in the form of observations, records of observable physical evidence of natural phenomena, and analyze this information to construct theoretical explanations of how things work. Knowledge in science is gained through research. The methods of scientific research include the generation of hypotheses about how natural phenomena work, and experimentation that tests these hypotheses under controlled conditions. The outcome or product of this empirical scientific process is the formulation of theory that describes human understanding of physical processes and facilitates prediction.

Lavoisier says, "... the impossibility of separating the nomenclature of a science from the science itself is owing to this, that every branch of physical science must consist of three things: the series of facts which are the objects of the science, the ideas which represent these facts and the words by which these ideas are expressed."

A broader modern definition of science may include the natural sciences along with the social and behavioral sciences, as the main subdivisions of science, defining it as the observation, identification, description, experimental investigation, and theoretical explanation of phenomena. However, other contemporary definitions still place the natural sciences, which are closely related with the physical world's phenomena, as the only true vehicles of science.

References:

http://en.wikipedia.org/wiki/Science