What's the difference between a coupling and a coupler?

What is the difference between a coupling and a coupler, terms are often used interchangeably in plumbing and mechanical contexts to mean device that joins two things, but there are subtle distinctions in usage. Coupling is noun that refers to fitting or device that connects two pipes, shafts, or components together in straight line, it is the physical part itself, for example pipe coupling joins two pipes, shaft coupling connects two shafts. Coupler is also noun but often refers to person or thing that couples, and in some industries coupler is used for quick connect device or adapter that allows coupling, for example garden hose coupler is quick connect that joins hose to faucet, or railway coupler connects train cars. In plumbing, coupling typically refers to permanent or semi permanent fitting like soldered copper coupling, while coupler may refer to quick disconnect or push fit connector that allows easy connection and disconnection. In electrical, coupler may refer to device that couples signals. In general, coupling emphasizes action or state of being joined, while coupler emphasizes device that does coupling, but in practice many people use terms interchangeably, especially in plumbing supply stores where coupling and coupler both mean connector that joins two pipes, though coupling more common for pipe fittings.