Zero

The concept that transformed mathematics from a tool for counting into a tool for describing the physical world — making algebra, calculus, and computing structurally possible, and sitting invisibly at the foundation of every calculation that modern civilization performs

Try to multiply MMDCCCXLVII by CCXCIV. There is no standard method. Roman numerals can record quantities and perform simple addition, but they cannot calculate in the notation itself — Romans computed on abaci and counting boards, then recorded results in a system that could not itself compute. Zero is not the number for nothing. It is the concept that makes the number system work.