Which of the following describes an API key?

An API key serves as an authorization token used to access APIs, which is essential for identifying the client making the request. When a client application interacts with an API, the API requires a way to verify that the request is made by a legitimate user or application. The API key acts as a unique identifier, allowing the server to determine what level of access the user has to the API's resources.

This mechanism is crucial for ensuring security and establishing a controlled environment for data exchange. It helps enforce limits on API usage and can aid in tracking usage patterns, enabling developers to monitor their applications effectively. The API key is usually sent with each API request, often as a query parameter or in the request header, depending on the API's design.

The other options discuss concepts that do not accurately represent what an API key is. Data encryption is concerned with securing data, query parameters are part of a URL used to send additional information to a server, and statistical measures of API performance assess how well an API functions in terms of speed, uptime, and reliability but do not pertain to the authorization aspect provided by an API key.