A professor may also mention the term "empirical" to students. In many cases, the term empirical is used to mean the exact same thing as a "primary" or "research." However, specifically, an empirical study is one that reports research based on direct observations or experiments. It often seeks to answer a specific question or to test a hypothesis.
The research may use quantitative research methods, generating numerical data which attempts to establish causal relationships between two or more variables. But empirical research articles may also use qualitative research methodology, in which the author objectively and critically analyzes beliefs or behaviors (etc.) without the analysis of clearcut numerical data.