Definition: The optimal condition is one that is the best for all things. In ergonomics that typically means that the task, work site, tool, etc. is the best size or shape for the majority of the user population.
Also Known As: Optimize
A one-size-fits-all baseball cap is the optimal size for a cap. It may not fit everyone (or anyone for that matter) the most comfortably but it will fit the majority of people within an acceptable level of comfort.