Scalability is the ability of a system, network, or application to handle a growing amount of work in a capable manner or its ability to be enlarged to accommodate that growth. In terms of JavaScript, scalability refers to the ability to quickly and efficiently increase the size and complexity of a website or web application without sacrificing performance or usability. This includes the ability to add more nodes (computers) to a distributed system, as well as the ability to easily add more features and capabilities to an existing system.