When paper is opaque it means that no light passes through it. If you hold a single sheet of paper up to a light source, and it's so thick that light won't shine through it, it's opaque.
Opaque paper is durable and strong. Cheaper paper tends to give off a dusty film when handled. Opaque paper has good finish. Therefore, it is primarily used in offices as office stationery and is ideal for typing, printing and photocopying.
Bond paper is opaque paper that is made with 20 to 100 percent cotton rag, which is what gives the paper its durable thickness and opacity.
A standard ream of opaque paper contains 500 sheets of 20-pound bond paper. The ream weighs approximately 5 lbs.