A terminal is a place where a wire comes out of an electrical component.
Terminal relations for the 2 wire devices studied so far (resistors, capacitors and inductors), describe the voltage across the two terminals and the current entering and leaving the terminals (which is always the same).
The terminal relations for a resistor are (called ohms law)
The terminal relations for a capacitor are
The terminal relations for an inductor are
A power supply doesn't have a terminal relationship or equation relating current and voltage. Either the current is constant (current supply) or voltage is constant (voltage supply). The power is varied depending upon what the circuit asks for.
One of the best places to build up your intuition or build your own animated circuit simulations is at falstad