insulator: A material which has a large energy gap is an insulating material. An insulator may conduct electricity; but only with a lot of effort --- notice that wood is a good insulator, but when lightning strikes a tree, this is electricity flowing through wood. Both semi-conductors and conductors conduct better than insulators.
interaction, gravitational: (See off-site: Mechanics in a Bar)
integral: Any quantity which can only have values equal to some integer (1, 2, 3...)
integral, half-: Any quantity which can only have values equal to half of some odd integer (1/2, 3/2, 5/2...)