What is the adhesive? What requirements must be satisfied for the adhesive?

Adhesion is that two solid materials can be bonded together through chemical force or physical force. The materials of adhesion are adhesive which can bond two substrates by tack.
It is common to name the natural matter or synthetic matter, organic matter or inorganic matter to bond two substrates as adhesive or glue for short. Adhesive is the general term including cement, glue, mucilage and mortar. Some terms can be general used in most occasions, such as physical states, chemical type, substrate and using conditions. Not every matter can be called adhesive, because the adhesive should have properties below.
①The adhesive must be fluid when used.
②The substrates can be fully wet by adhesives.
③The adhesive must form adhesive layer from fluid to solid.
④The adhesive must have the properties of passing stress, impact resistance and good bonding strength.
⑤The adhesive must be long-term effective.