"Contamination of oil is any excess or deficit of a substance or a form of energy which has an adverse effect on the operation, the life or the reliabity of a system."
Source : The management of oil contamination in hydraulic equipment by Dr. Ir. P. Blok
(Autorized translation of the partly revised second Dutch edition - ISBN-909008458-4)
According to Dr. Ir. Peter Blok, contamination is not restricted to the presence of insoluble solid particles in oil.
Dissolved substances and various forms of energy, if present in amounts that exceed or fall below a certain limit, also count as contamination. For example, oil temperature and system pressure may turn out to be contaminants in this sense. However, system pressure problems only occur infrequently or can be avoided by simple design measures.
We will focus on the main types of contamination: solid particles, water, air, chemical and thermal pollution.
PS. check out the white paper Introduction to Oil Cleanliness and learn more on the importance of clean oil!