A vehicle that was not produced to US Standards and then imported to the US. Some legally, IE brought up to all US standards. Bumpers, headlights, cats, side door reinforcement etc.. They were done to varying degrees by the importer. The following are 107 models that can be found in the US but are all grey market.
280SL, 300SL, 420SL and 500SL.
Some, like the 380SL were made for most markets so a 380SL can be a US version or a grey market.
I have an '81 280SL that has no US updates. I have an '84 500SL that had an AIR pump and cats. It was imported into Texas and modified there.

The euro versions are more desirable to some than the US because they generally have higher compression and HP. Some also have the manual trans.