Nonionic substances are defined as:

The definition of nonionic substances refers specifically to their behavior in a solution, particularly in terms of their electrolyte properties. Nonionic substances do not dissociate into free ions when dissolved in a solvent, which means they remain as complete molecules without breaking apart into charged ions. This characteristic makes them different from ionic substances that do form free ions.

In a medical context, especially in imaging and radiology, nonionic contrast agents are preferred in many situations as they tend to be better tolerated by patients, resulting in fewer adverse reactions compared to ionic contrast agents that can produce free ions in the bloodstream.

The other options address specific types of substances or characteristics but do not encapsulate the fundamental definition of nonionic entities in a solution. Barium-based and iodine characteristics refer to specific types of contrast agents, while “noniodinated” concerns the presence of iodine specifically, which is not universally applicable to all nonionic substances.