Land surrounded by water. An island is defined by its separation as much as by its ground.
Old English igland, from ieg (island, watery place) + land; a native compound whose modern spelling was reshaped in the 16th century by the unrelated French isle, from Latin insula, though the pronunciation never changed.
Ground defined as much by what separates it as by what it is made of — an island without its water would just be more ground.