Common Lisp the Language, 2nd Edition
Streams are objects that serve as sources or sinks of data. Character streams produce or absorb characters; binary streams produce or absorb integers. The normal action of a Common Lisp system is to read characters from a character input stream, parse the characters as representations of Common Lisp data objects, evaluate each object (as a form) as it is read, and print representations of the results of evaluation to an output character stream.
Typically streams are connected to files or to an interactive terminal. Streams, being Lisp objects, serve as the ambassadors of external devices by which input/output is accomplished.
A stream, whether a character stream or a binary stream, may be input-only, output-only, or bidirectional. What operations may be performed on a stream depends on which of the six types of stream it is.