diff --git a/collects/scribblings/guide/io.scrbl b/collects/scribblings/guide/io.scrbl
index 765b020113..75c2066666 100644
--- a/collects/scribblings/guide/io.scrbl
+++ b/collects/scribblings/guide/io.scrbl
@@ -16,11 +16,17 @@
 
 @title[#:tag "i/o" #:style 'toc]{Input and Output}
 
-A Racket @deftech{port} represents an input or output stream, such as
-a file, a terminal, a TCP connection, or an in-memory string. More
-specifically, an @defterm{input port} represents a stream from which a
-program can read data, and an @defterm{output port} represents a
-stream for writing data.
+
+@margin-note{A Racket port corresponds to the Unix notion of a stream
+(not to be confused with @racketmodname[racket/stream]'s streams).}
+A Racket @deftech{port} represents a source or sink of data, such as a
+file, a terminal, a TCP connection, or an in-memory string.  Ports
+provide sequential access in which data can be read or written a piece
+of a time, without requiring the data to be consumed or produced all
+at once.  More specifically, an @defterm{input port} represents a
+source from which a program can read data, and an @defterm{output
+port} represents a sink to which a program can write data.
+
 
 @local-table-of-contents[]