An API (Application Programming Interface) allows applications to talk to one another and can be used to facilitate downloading large amounts of data from a website. APIs are useful when you want to extract a large amount of data programmatically for text or data mining.
Create and manage an API key in the API Key Management section in your My NCBI Account Settings.
E-utils users are allowed 3 requests/second without an API key. Create an API key to increase your e-utils limit to 10 requests/second.
Contact the NCBI help department if you need to request higher throughput. Only one API Key per user. Replacing or deleting will inactivate the current key. Use this key by passing it with api_key=API_KEY parameter. Refer to additional documentation for more information.