Enerdhil was a legendary, and quite possibly mythical, smith of Gondolin. According to one account, it was Enerdhil who made the original Elessar, an astonishing green gem through which marred and broken things could be seen as though whole and healed again. This Elessar was given as a gift to Idril Celebrindal, and given by her to her son Eärendil, who bore it on his voyages across the Great Sea.
Enerdhil's name has not been explained. It's possible that the last part of his name includes dîl "stopper" but even this is not certain.
Other versions of the legendarium
The fact that there are two possible Elessars, makes Enerdhil fit into the mythos, as the first one could had been made by him, and the second by Celebrimbor. But not even this have been resolved clearly.