During the early development of C, one of the design decisions made was to move file input and output out of the language and into a series of standard library function calls. This application programming interface (aka API) is known as stdio.
Unfortunately, in the beginning, no formal specifications for stdio were available. Instead, the API and a brief description existed, and, for those fortunate to have access to the original Bell Labs source, an implementation in C was available.
However, the basic concept is this.
Unix borrowed the standard application files from Multics, which had longer names for them (standard_input, standard_output, and error_output). - escargo
See also