Like any discipline, psychology has developed its own language. There are terms with specific meanings to a psychologist that might have a different meaning to a layperson. For instance, schizophrenia is a diagnostic label relating to people with inappropriate affect and behavior, but to the general public, Schizophrenia means a split personality (which a psychologist would call dissociative identity disorder).
Similarly, in different areas of psychology, terms may not reflect the same concepts. For instance, different psychologists who see the abbreviation SSRI could interpret in differently. Within the realm of treatment for psychological disorders, those initials stand for selective serotonin reuptake inhibitors, whereas in studies of emotional intelligence, the initials refer to the Schutte Self-Report Inventory.
In your introduction, you can let your reader know how you are using important terms. Sometimes you might be discussing a relatively obscure concept that is generally known within a limited domain of psychology, such as Schutte’s inventory. Your reader might need help understanding the concept. If you are not making use of any unusual terms or definitions, you probably don’t need to clarify your terminology, but you can let your reader know about any such instances here.
In the same vein, you can use your introduction to discuss the operational definitions that previous researchers have used if you believe you can generate different and better operational definitions or if there is disagreement among professionals about how to measure a construct. As an example, in studies of emotional intelligence, it appears to make a difference how the researchers measure this concept (Zeidner, Shani-Zinovich, Roberts, & Matthews, 2005).