what do you mean by empirical

In order for a theory to be proved or disproved, empirical evidence must be collected. An empirical study will be performed using actual market data. For example, many empirical studies have been conducted on the capital asset pricing model (CAPM), and the results are slightly mixed.
Empirical probability uses the number of occurrences of an outcome within a sample set as a basis for determining the probability of that outcome. The number of times “event X” happens out of 100 trials will be the probability of event X happening. An empirical probability is closely related to the relative frequency of an event.

This has to do with the use of the empirical precepts in practical instruction.
Here the great value of the empirical knowledge of the voice is seen.

Review articles, sometimes called literature reviews or secondary sources, synthesize or analyze research already conducted in primary sources. They generally summarize the current state of research on a given topic.
Research articles, sometimes referred to as empirical or primary sources, report on original research. They will typically include sections such as an introduction, methods, results, and discussion.

References:

http://www.thesaurus.com/browse/empirical
http://jwu-ri.libanswers.com/faq/141979
http://www.merriam-webster.com/dictionary/foreword