- 1a : any of various strong adhesive substances; especially : a hard protein chiefly gelatinous substance that absorbs water to form a viscous solution with strong adhesive properties and that is obtained by cooking down collagenous materials (as hides or bones)
- b : a solution of glue used for sticking things together
- 2: something that binds together <enough social glue…to satisfy the human desire for community — E. D. Hirsch, Jr.>
Glue or an adhesive is any substance that, when applied to the surfaces of materials, binds the surfaces together and resists separation. The term "adhesive" may be used interchangeably with glue, cement, mucilage, or paste. Adjectives may be used in conjunction with the word “adhesive” to describe properties based on the substance's physical form, its chemical form, the type of materials it is used to join, or the conditions under which it is applied.
The use of adhesives offers many advantages over other binding techniques such as sewing, welding, bolting, screwing, etc. These advantages include the ability to bind different materials together, the ability to distribute stress more efficiently across the joint, the cost effectiveness of an easily mechanized process, an improvement in aesthetic design, and increased design flexibility. Disadvantages of adhesive use include decreased stability at high temperatures, relative weakness in bonding large objects with a small bonding surface area, and greater difficulty in separating objects during testing.
Adhesives may be found naturally or produced synthetically. The earliest use of adhesive-like substances by humans was approximately 200,000 years ago. From then until the 1900s, increases in adhesive use and discovery were relatively gradual. Only since the last century has the development of synthetic adhesives accelerated rapidly, and innovation in the field continues to the present.
The earliest use of adhesives was discovered in Italy. At this site, two stone flakes partially covered with birch-bark-tar and a third uncovered stone from the Middle Pleistocene era (circa 200,000 years ago) were found. This is thought to be the oldest discovered human use of tar hafted stones.