Question: Scenario: power outlet in the yard for electric vehicle, to avoid running an ugly extension from inside home and leaving an open window; I need to avoid that anyone just comes and uses my power though.

Enter the switch I'm looking for.

This would be the operational flow:

1. Attach the charging cable of the vehicle to the outdoor outlet (no current will flow since the outlet is not powered - yet)
2. get inside, arm said switch, current will start flowing as the vehicle begins charging, and the switch will stay on
3. Either the vehicle stops charging, or I unplug it, or someone else does because they want to use my energy for free: the switch will detect no more current flowing through it, and will disarm, leaving the outdoor outlet inactive

I found two somewhat similar questions:

[Need a switch/circuit breaker that stops power supply to equipment when power comes back after outage](https://electronics.stackexchange.com/questions/384591/need-a-switch-circuit-breaker-that-stops-power-supply-to-equipment-when-power-co) (not good for me, since it is intended to check for incoming power, not outgoing current flow)

[I'm looking for a circuit breaker that interrupts the circuit when current drops to zero](https://electronics.stackexchange.com/questions/487375/im-looking-for-a-circuit-breaker-that-interrupts-the-circuit-when-current-drops) (not good, as it is for DC rather than AC)

Is there a name for this peculiar kind of switch, and/or is DIY easily accessible?

Answer: 
There are control panels for pool/submersible pumps with dry run protection (switch off when current is below a setup value). Maybe you find some one-phase starter that can fit your needs.
Install a security camera. What you describe is definitely theft and is prosecutable. The presence of a camera should be enough to dissuade folks anyhow. This isn't like porch pirates who can come and go in a few seconds...someone using your juice would be vulnerable to detection for quite a long time.