Scalability means the ability to handle growth in either of these 2 areas:
1) Ease of adding new features, fixing bugs. To make the code do these new things how hard is it to do? While this may be maintainability in some ways, it is also the question of whether you can build the code to handle a bigger scale easily.
2) Ability to handle greater and greater load which may be multiple requests simultaneously in Web Development or larger datasets for a simple Database-driven website, e.g. could you make an application that handles tables with hundreds of thousands of rows that is still viewed as "quick"?
Generally for me scalability means the ability to grow.
