Definition
A Flight Simulation Training Device whose cockpit and software can be reconfigured to represent more than one aircraft type or model. The device is qualified separately for each configuration it can be set to, and only one configuration is active and approved for use at any given time.
Plain English
A flight simulator that can be switched between different aircraft setups. It might be set up as one airplane today and a different one tomorrow, but it is only ever used as one aircraft at a time, and each setup has to be approved on its own.
Context Anchor
Seen in simulator training, training-center records, and approval documents that identify which aircraft setup the device may be used for.
Derivation
Convertible comes from Latin convertere, 'to turn around' or 'change.' The simulator is literally turned from one aircraft configuration into another. FSTD is the FAA's umbrella term covering full flight simulators and flight training devices.
Why Pilots Care
It lets a single device support type-specific training for several aircraft, lowering cost while still meeting regulatory requirements for each model.
Intuition Check
Convertible does not mean a car with a folding roof here. It means the simulator can be changed from one aircraft setup to another approved setup.
Example Sentence 1
The training center's convertible FSTD was reconfigured overnight from the 737 setup to the A320 setup for the next morning's class.
Example Sentence 2
Before the checkride the examiner verified that the convertible FSTD was correctly configured for the specific aircraft variant being tested.