Stream definition

Java programs perform I/O through streams. A stream is an abstraction that either produces or consumes information. A stream is linked to a physical device by the Java I/O system. All streams behave in the same manner, even if the actual physical devices to which they are linked differ. Thus, the same I/O classes and methods can be applied to any type of device. This means that an input stream can abstract many different kinds of input: from a disk file, a keyboard, or a network socket. Likewise, an output stream may refer to the console, a disk file, or a network connection. Streams are a clean way to deal with input/output without having every part of your code understand the difference between a keyboard and a network, for example. Java implements streams within class hierarchies defined in the java.io package.

From: Java The Complete Reference

A stream is a concept, but it's not that strict, that just only one description would be correct.

An I/O Stream represents an input source or an output destination. A stream can represent many different kinds of sources and destinations, including disk files, devices, other programs, and memory arrays. Streams support many different kinds of data, including simple bytes, primitive data types, localized characters, and objects. Some streams simply pass on data; others manipulate and transform the data in useful ways.

No matter how they work internally, all streams present the same simple model to programs that use them: A stream is a sequence of data.

From: http://download.oracle.com/javase/tutorial/essential/io/streams.html

Also a stream is either an input stream or output stream. If it is an input stream, in Java it will adhere to the InputStream interface, the latter to the Outputstream.

(Side note: In crypto, there's e.g. a difference between stream and block ciphers, where a stream cipher is something that does not know (in a very general sense) anything about the future, while a block cipher knows its (maximum) size in advance and the sizes of all coming blocks.)

I would say a Stream is like all of these, but not exactly any of these.

I would say its an ordered sequence of 8-bit bytes.