When an action potential is propagating through a neuron, it seems to me that the time for "potential of action" is over, and that we are now just in a state of "action". Why don't we just call action potentials "action"? Where is the potential for action when the neuron itself is undergoing action? In fact, when a neuron is at rest, it is said to have a "resting potential". To me this seems more like a "resting state" and an "action state". So what is so potential about "action potentials"?
There are other meanings of "potential" than the regular meaning "having or showing the capacity to become or develop into something in the future" (source: Google Translate).
In case of "resting potential" it refers to the voltage difference across the cellular membrane. Every cell maintains a voltage difference across the membrane (see a related question here: https://biology.stackexchange.com/questions/17840/resting-and-action-membrane-potential). The voltage difference is used for various purposes, including the transport of nutrients and ions, and it is crucial for action potentials to occur.
In case of "action potential" it refers to a series of successive changes in the voltage difference across the membrane, which occurs when an excitable cell is triggered (see a related question here: https://biology.stackexchange.com/questions/91/how-do-the-brain-and-nerves-create-electrical-pulses/110#110).