Yamaimo (ヤマノイモ) literally means "mountain potato," and it's a member of the yam family which grows a long tuber with whiskers that has tan skin and white flesh. The unique thing about yamaimo is that it has a thick mucilaginous texture when grated raw, which is very similar to raw egg. When heated, the starch in the yam gels, giving it the ability to trap some air as well. This is what makes it such a good egg substitute.  Traditional okonomiyaki recipes usually call for both yamaimo and egg, but yamaimo is perfectly capable of binding the pancake together without the help of egg. The texture does end up a little more dense and moist, but I actually prefer this as it becomes similar to takoyaki.