An ordened set of words and word groups of which the form and interrelations have been documented (ISO 2788).
A thesaurus is based on knowledge that already existed before the document was drawn up (a priori); it does not represent insights into the relationships between the concepts and knowledge achieved on the basis of the document (a posteriori). In other words, a thesaurus will not provide any new knowledge (but can, however, be instructive and useful.)
Classifications and thesauri are purely mental constructs. Classes and preferred terms are a reflection of current knowledge and preferences; they form elements of a scientific debate, but are not universal and certainly not eternal in nature.