The source is the farthest point of the river stream from its estuary or its confluence with another river or stream. Rivers are usually fed by many tributaries. The farthest stream is called the head-stream or head water. There is sometimes disagreement on which source is the head water, hence on which is the true source.
A lake fed by many rivers is sometimes called the source of the bigger river flowing out of it. For example, Lake Victoria is often called the source of the Nile, as the rivers that flow into it have names of their own.
Headwaters are usually in mountains. Glacial headwaters are made by melting of glaciers.