API

An application programming interface (API) is a defined way for one program to request data or services from another. NIST's glossary defines it as "A system access point or library function that has a well-defined syntax and is accessible from application programs or user code to provide well-defined functionality" (NIST SP 1800-16C, drawing on NISTIR 5153). An interface, in the general sense, is a "Common boundary between independent systems or modules where interactions take place" (CNSSI 4009-2022).

What an API changes

A person uses a website one page at a time. A program uses an API to ask for records directly and receive them in a structured form it can process. The Library of Congress describes this as "Computational access": the loc.gov site makes information available "via its JSON API, sitemaps, and suite of microservices."

A government example

For its Chronicling America newspaper collection, the Library of Congress says each of its data views "uses common Web protocols," and "Together they make up an extensive application programming interface (API) which you can use to explore all of our data in many ways." The loc.gov API "is accessible to the public with no API key or authentication required, however, rate limiting is enforced." Requests "that exceed the rate which loc.gov can successfully accommodate will be blocked to prevent a denial of service."

Keys and rate limits

Many federal APIs require a key. api.data.gov, "a free API management service for federal agencies," handles "API keys, rate limiting, and gathering usage stats" for participating agencies. It states that "Users can reuse their API key across all participating api.data.gov APIs" and that "Individual users can be given higher or lower rate limits." The purpose of limits, in its words: "Prevent abuse."

APIs and AI

NIST's machine learning glossary describes the deployment stage as the point where a model is put into use, "such as being integrated into an enterprise application or made available to end users through an API" (NIST AI 100-2e2025). Many AI tools, including large language models, are reached this way.

An API is often the collection step of an ETL job or a Data Pipeline.

Why it matters for land and mining claim records

An API returns what the provider's system holds at the moment of the request, under the provider's limits. A pull that hits a rate limit, times out, or pages through results can come back incomplete without an obvious error. A research file should record when data was pulled, from which service, and how many records came back, so the result can be checked or repeated.