It's surely 'in the eye of the beholder' and your Golden Tang is absolutely a delightful specimen plant if ever there was one.
How about "A nice, full mature plant that blooms generously on multiple spikes" or even just "any plant that you'd want to show when it's blooming" ?
As I said earlier, I'd consider my big noid Catt to be a specimen plant, and I doubt I'll ever show any of my orchids. But you never know.
On this train of thought, is it my imagination or do orchids get more robust and healthy the bigger they get? They seem to develop better disease resistance and also grow more new pb's each year on bigger plants. Or maybe it's that the plants that grow into big specimens are just robust in the first place . . . uh oh, it's the chicken and the egg isn't it!
Must be getting late.