(1) A popular buzzword that refers to how well a hardware or software system can adapt to increased demands. For example, a scalable network system would be one that can start with just a few nodes but can easily expand to thousands of nodes. Scalability can be a very important feature because it means that you can invest in a system with confidence you won't outgrow it.
(2) Refers to anything whose size can be changed. For example, a font is said to be scalable if it can be represented in different sizes.
(3) When used to describe a computer system, the ability to run more than one processor.