The term 'Fundamental Defect' can be summarized as a major FLAW or deficiency. It is often used to describe a problem that is so severe that it cannot be easily fixed or remedied. This term is commonly used in legal contexts, where it is used to refer to a defect in a contract or other legal document that renders it invalid. In the crossword, the solution for this term was 'FLAW', which is another word for a major defect or mistake.