Empirical method is generally meant as the collection of data on which to base a theory or derive a conclusion in science. It is part of the scientific method, but is often mistakenly assumed to be synonymous with the experimental method.
The empirical method is not sharply defined and is often contrasted with the precision of the experimental method, where data are derived from the systematic manipulation of variables in an experiment. Some of the difficulty in discussing the empirical method is from the ambiguity of the meaning of its linguist root: empiric.
According to the Oxford English Dictionary (2nd Edition, 1989), empiric is derived from the ancient Greek for experience, έμπειρία, which is ultimately derived from έυ in + πεἳρα trial, experiment. Therefore, empirical data is information that is derived from the trials and errors of experience. In this way, the empirical method is similar to the experimental method. However, an essential difference is that in an experiment the different "trials" are strictly manipulated so that an inference can be made as to causation of the observed change that results. This contrasts with the empirical method of aggregating naturally occurring data.
Adding further confusion is another connotation of empiric. Strict empiricists are those who derive their rules of practice entirely from experience, to the exclusion of philosophical theory. An example of this is the derivation of the psychopathology scales of the MMPI questionnaire, where items that were endorsed by people in various diagnostic categories were used as designation to that diagnosis irrespective of any understanding of the link between the item and the pathology.
The OED further states that an empiric is "one who, either in medicine or in other branches of science, relies solely upon observation and experiment" [emphasis added]. In this case, an empiricist can be someone who conducts an experiment but without using a hypothesis to guide the process, i.e., strictly by the trial-and-error method. This is counter to one of the main tenets of the scientific method, that of the hypothetico-deductive method, where the manipulation of the variable in an experiment is dictated by the hypothesis being tested.
Thus we arrive at better understanding of a standard definition for the empirical method from AccessScience@McGraw-Hill:
[The empirical method] is generally characterized by the collection of a large amount of data before much speculation as to their significance, or without much idea of what to expect, and is to be contrasted with more theoretical methods in which the collection of empirical data is guided largely by preliminary theoretical exploration of what to expect. The empirical method is necessary in entering hitherto completely unexplored fields, and becomes less purely empirical as the acquired mastery of the field increases. Successful use of an exclusively empirical method demands a higher degree of intuitive ability in the practitioner.