Research studies usually include terms that must be carefully and precisely defined, so that others know exactly what has been done and there are no ambiguities. 

Sometimes the definitions themselves are not important, provided a clear definition is given. Sometimes, commonly-accepted definitions exist, so should be used unless there is a good reason to use a different definition (for example, in criminal law, an ‘adult’ in Australia is someone aged 18 or over, which is different from Indonesia).

Sometimes, a commonly-accepted definition does not exist, so the definition being used should be clearly articulated.

Two types of definitions can be given: conceptual definitions and operational definitions.

Example 1

Topic: Water temperature. 

Conceptual definition: What is meant by ‘temperature.’ 

Operational definition: How the ‘temperature’ would be measured: the thermometer type, how the thermometer was positioned, how long was it left in the water, and so on.

Example 2

Topic: Stress among university students. 

Conceptual definition: What is meant by ‘stress.’ 

Stress is a feeling of emotional or physical tension. It can come from any event or thought that makes you feel frustrated, angry, or nervous. Stress is your body's reaction to a challenge or demand. In short bursts, stress can be positive, such as when it helps you avoid danger or meet a deadline.

Operational definition: How the ‘stress’ would be measured.

Stress cannot be measured directly, but could be assessed using a survey (like the Perceived Stress Scale (PSS) (Cohen et al. 1983)).

The operational definition of stress is the score on the ten-question PSS. Other means of measuring stress are also possible (such as heart rate or blood pressure).

Example 3

Topic: The Effect of Lean Manufacturing Implementation on Operations Performance. 

Conceptual definition: 

Lean manufacturing is an approach synergistically addressing to improve operations performance and business performance through waste elimination. 

Operations performance comprises actual outputs of operations strategies employed, which is influenced by operating conditions and represents some internal properties of manufacturing system.

Operational definition: 

Lean manufacturing is measured by pull system, small lot production, flexible resources, cellular layouts, quick setups, supplier networks, etc. 

Operations performance is measured by quality, flexibility, productivity, cost reduction, inventory minimization, lead time, safety, environmental performance.