I've heard this term many times from many different sources, but none of them gave me a definition.
Here is my understanding: "coupled" means "connected together". If two circuits are "capacitively coupled", that means they are connected with capacitors between them.
However, one question remains: What is an "ac coupled signal"? I know circuits can be coupled, but how can we couple a signal?
The manner of "coupling" is referring to what signals can pass. An AC coupled system is also a DC blocked system. Similarly, a DC coupled system allows both the DC and AC components of the signal to pass through the system.
The relationship between AC coupled and capacitively coupled is in the use of a capacitor to achieved the DC blocking ability of the system. By inserting a capacitor in series with the signal, a slowly changing signal can no longer pass. This is a high-pass filter.
An example of this is the DC/AC/GND selection of an oscilloscope input. In DC coupled mode, there is no filtering other than that inherent in the probe/cable/input, meaning you can measure slowly changing signals and (inaccurately) measure DC voltages. In AC coupled mode, a capacitor is switched in series inside the oscilloscope, creating a high-pass filter, meaning you can more easily measure signals with a large DC offset. This is useful for measuring the small, fast transient of a switching power supply experiencing an output step load change (with a large DC offset) or many other signals. Finally, in GND mode, the internal switch disconnects the external input and grounds the internal circuitry.
