The term 'grand number' is a mathematical concept that refers to a number that is greater than all other numbers in a given set. The two crossword solutions for this term are THOU and IOOO, both of which are four-letter words. THOU is an archaic form of the word 'THOUSAND, ' while IOOO is an alternate spelling of the Roman numeral '1000.' Both of these words refer to large numbers, making them appropriate solutions for the term 'grand number.'