The term 'BIG REF.' is an acronym for 'Big Reference'. It is a term used to describe a large reference book, such as a DICTIONARY or encyclopedia. The crossword solutions for this term are 'DICT' (4 letters) and 'OED' (3 letters). DICT stands for 'DICTIONARY', which is a book that contains a list of words in alphabetical order, along with their definitions. OED stands for 'Oxford English DICTIONARY', which is a comprehensive DICTIONARY of the English language published by Oxford University Press.