What is the difference between electric potential, potential difference, and voltage? I see both terms being used from time to time. Are they interchangeable?
 A: Electrical potential is the energy per unit charge gained or lost when a charge is moved from some reference point at which the potential is defined to be zero.
Voltage is the difference in potential between two arbitrary points at which the potential is not necessarily zero.
For example, the potential at point A relative to a defined reference point (sometimes an infinite distance away) might be 10000 volts, and the potential at point B might be 10100 volts. Then the voltage between B and A is 100 volts.
While they represent the same type of quantity, they have slightly different connotations.
A: Being a little pedantic, the electric potential is an absolute number, after choosing your gauge, of course. 
On the other hand, "voltage" is actually a DIFFERENCE of two potentials. The difference of the potential at one point and another. But we usually set "0 volts" the reference, so they are used interchangably.
A: In my experience, "voltage" is not a formal term, and therefore never gets a formal definition. However, it is essentially used in practice to mean "what we measure with a voltmeter." This is an operational definition.
If you use a voltmeter in a static field or a DC circuit, then there is a well-defined potential, and the voltmeter measures the difference in potential.
An oscilloscope is also a kind of voltmeter. Let's say you use an oscilloscope to measure the EMF induced in a coil by a changing magnetic field. Then the thing we're measuring does have units of volts, and can probably be called a voltage, but it is not a difference in potential. There is no well-defined electric potential in this case.
