An application programming interface (API) is a set of definitions, protocols and functions that control how one application interacts with another application.
APIs are fundamental to the functioning of software and devices on the Internet. Historically, APIs have been considered primarily functional and thus not subject to significant protection under U.S. copyright law.
The current copyright litigation between Google and Oracle over the Java API has determined that APIs can be protected under copyright, but Google’s use of more than 11,500 lines of Java code has been ruled “fair use” and thus not considered copyright infringement.
The breadth of Google’s non-infringing fair use means that relying on copyright to protect an API is uncertain at best.
Patents, on the other hand, are intended to protect the functional aspects of a product or process. Implementations of patent eligible processes using an API are, therefore, generally protectable under U.S. patent law. Thus, developers wishing to protect their investment in a valuable API may still need to consider patent protection.