What is the difference between mental workload and mental stress?

The difference between stress and strain in occupational psychology lies in their definition and effect:

Stress describes all external, objective factors that affect a person in the workplace. Stress can be of a physical, psychological or social nature and includes, for example, the working environment (noise, temperature), the work tasks (difficulty, complexity) and the work organization (working hours, break arrangements). Stress is therefore that which comes to the person from outside and can initially be viewed neutrally.

Stress, on the other hand, describes the individual, subjective reaction to these stresses. It depends heavily on the person's personal condition, experience, knowledge and abilities. A certain load can therefore lead to different levels of stress depending on the person. Strain can manifest itself in the form of stress, fatigue, frustration or even positive challenges and is the actual effect that the strain has on the person.

To summarize:

Stress = external working conditions
Stress = individual impact of these conditions on the person