API (application programming interface)

A defined way for one piece of software to request data or services from another, allowing different applications to connect and share information automatically.

APIs are the connectors behind much of modern business software. When your point-of-sale system sends sales to your accounting software, or your website pulls appointment slots from a booking platform, an API is usually doing the work. Access to an API is typically controlled by keys or tokens that act like passwords for software.

APIs matter for security because they can expose data directly, without anyone going through a normal login screen. A leaked API key, an overly permissive integration or a poorly secured custom connection can let an outsider pull customer records or financial data. Small businesses often connect many cloud apps together over time and lose track of what has access to what.

Practical tip: keep a list of the integrations connected to your key systems, such as Microsoft 365, Google Workspace and your accounting platform. Remove ones you no longer use, never share API keys by email or store them in shared documents, and rotate keys when a developer or vendor relationship ends.