In computers, scalability is how well a system can grow when more and more people start using it. A site with good scalability still runs smoothly if ten people visit, or ten thousand. Engineers talk about scaling up (a bigger machine) and scaling out (more machines working together). The word is fairly formal; you will meet it at work, in start-ups, and in IT class more than in daily chat. Though actually, people now use it for businesses too — a company with scalability can get much bigger without falling apart. Same idea, just not about servers.