Stream means little river in everyday understandings but in Geography it means any channel that water flows in. Stream order indicates the position of the channel in the drainage system and its relative size.
All streams start as first-order streams. These are not fed by any other streams and are usually found in the upper reaches of a drainage basin.
When two first-order streams join, they become a second-order stream. Two second-order streams join to form a third-order stream and so on.
If a first-order stream joins a second-order stream, the stream remains a second order stream.
IMPORTANT: the stream order only increases if two streams of the same order join.