Adhesive, any substance that is capable of holding materials together in a functional manner by surface attachment that resists separation. “Adhesive” as a general term includes cement, mucilage, glue, and paste—terms that are often used interchangeably for any organic material that forms an adhesive bond. Inorganic substances such as portland cement also can be considered adhesives, in the sense that they hold objects such as bricks and beams together through surface attachment, but this article is limited to a discussion of organic adhesives, both natural and synthetic.