Refers to the ability of hardware or software or even a brand, to adapt to increased demands while continuing to work accurately. It also describes how well a solution to some problem will work when the size of the problem increases at a later time. For example, in the industry, developers are commonly concerned with building Web sites that are scaleable and with setting up an infrastructure that is able to grow.
Scalability also refers to the economies of scale and scope the Internet provides; for example, once a company creates a "category killer" in a particular niche, it's possible to scale up and capture that market worldwide at little additional cost. Slang usage can refer to your job, as in, "Dude, why would you take a job designing at a newspaper? It's so not scaleable."