There are many definitions of what makes an island, but one in particular still seems to hold sway:
"the 1861 Census decreed that an island is 'any piece of solid land surrounded by water which affords sufficient vegetation to support one or two sheep, or is inhabited by man'."
BERRY, R. J., 2009. Islands.
This works for us. It makes it easier to find the cut-off point between a rock and an island. M. Shea agrees:
"If a piece of land will support a sheep, it is an island; if it will not, it is a rock."
SHEA, M. 1981. Britain's Offshore Islands