A stream is a series of data elements (characters, bytes or complex packets) which can be accessed or made accessible in a serial fashion. Random access is not possible.
The term stream is also used for streaming media, a method in which the media can be played while it is being accessed (no full download required). The ability to stream media is largely dependent on the container format used. Seeking functionality in streaming media is typically achieved through a different protocol, for example RTSP or HTTP 1.1's Range function.