In circuit analysis, it refers to an ideal generator whose voltage is independent of the current through it, or whose current is independent of the voltage across it.
In simple linear circuit analysis, a more practical two-terminal circuit can be reduced to either of these two cases, which are equivalent.
1. Thévenin-equivalent circuit: an ideal voltage generator in series with a resistor to the output. The voltage of the internal generator equals the value measured with an open circuit across the output.
2. Norton-equivalent circuit: an ideal current generator in parallel with a resistor across the output. The current of the internal generator equals the value measured through a short circuit across the output.
The equivalent resistors in the two cases for an individual circuit are equal.
In power supplies, compare the output resistance ROUT to the actual load resistance RLOAD connected to the output terminals.
If ROUT << RLOAD, that is a good constant-voltage source.
If ROUT >> RLOAD, that is a good constant-current source.
For amplifiers and similar circuits, a good rule-of-thumb is to connect a constant-voltage source to a high-resistance load, and a constant-current source to a low-resistance load (or input of the following amplifier stage).