Coupling is a term widely used in discussing wireless power systems – it refers to a coupling coefficient ‘k’ which defines how well a transmitter and receiver are magnetically “linked” as a percentage. Generally something like transformers have extremely high coupling coefficients approaching 100%.
Technically, tightly coupled systems are interpreted as having high or transformer like coupling coefficients while loose coupling is interpreted as systems with low coupling coefficients. There is some debate around the cut-off for high and low – 50% is one proposed transition point.
From a usability perspective I would define tightly coupled as a system which requires some form of mechanical alignment to fix orientation and transmission distance. This can be done via a magnet or a mechanical alignment feature on the transmitter and receiver. A loosely coupled system would allow complete flexibility of orientation / misalignment in a 2D target zone. A 2D target zone would be a planar area such as a matt (i.e. Proxi-2D), which can wirelessly charge devices imbedded with a receiver that is a few cm above it.
Systems like the Proxi-3D which enables receivers to work in a three dimensional target zone with an omni-directional receiver are what a user may call as beyond loosely coupled!
In my opinion the usability definition is more relevant for anyone buying the system as really the technical definition is really just a means to an end – the end being ensuring customers don’t have to carefully align their devices with a wireless charger.