API stands for application programming interface. Sadly for linguistic purists it has become a geekspeak verb; so banks can "API into another bank" or, as one user puts it: "We just API what we need." But API matters. Crudely put, it is a method of allowing programmers to develop additional functionality to boost the capabilities of a given piece of software. It is a way for banks to retain proprietary control over off-the-shelf products, and can mean that a lesser product can rival a more expensive one after the in-house programmers have got their hands on it. Any product worth its salt in the financial technology world (and far beyond - Google, for example, offers API tools) will give its users the opportunity to build on top, thereby combining the best of both buy and build worlds.