API security is a set of security measures to protect an application programming interface (API) from malicious attacks. It involves authentication, authorization, input validation, encryption and logging. Authentication is the process of verifying the identity of users before they can access the API. Authorization ensures that only authorized users are allowed to access the API. Input validation prevents malicious data from entering the system. Encryption ensures that data is securely transmitted between the client and the server. Logging helps to detect and investigate any suspicious activity.