The system of symbols — numerals, operators, variables, and logical signs — through which mathematics is written, communicated, and extended: not mathematics itself, but the technology that made mathematics scalable, shareable, and capable of reasoning beyond what any individual mind could hold.
Mathematics existed before mathematical notation. But it was trapped in individual minds and spoken language. Notation is what freed it: the technology that allowed ideas to be stored outside the mind, checked by others, built upon across generations, and extended to problems no single person could solve alone. The equals sign is as much a piece of infrastructure as a road.