An API rate limit is a restriction set by a service provider that determines how many times a user or application can call an API in a specified time period. This limit helps protect the service from being overwhelmed by too many requests, ensuring fair usage among all users. By limiting requests, providers can maintain performance and prevent abuse that can lead to service degradation. Rate limits can be set on various levels, including user accounts or IP addresses, and often vary depending on the nature of the API service.