A stream is an object that can be used as a source or sink for characters—either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a *Help* buffer, or to the echo area.
nil, in addition to its other meanings, may be used
as a stream. It stands for the value of the variable
standard-output. Also, the object
t as a stream specifies input using the minibuffer
(see Minibuffers) or output in the echo area (see The Echo Area).
Streams have no special printed representation or read syntax, and print as whatever primitive type they are.
See Reading and Printing Lisp Objects, for a description of functions related to streams, including parsing and printing functions.