Any substance used for making a negative mould of a structure. Rigid impression materials are inflexible and are usually used to take impressions of structures which have no undercut surfaces such that when removed the material is not required to be deformed; such materials include plaster of Paris, impression compound, zinc oxide, and wax. Elastic impression materials are capable of deformation with minimal distortion; materials commonly used are reversible and irreversible hydrocolloid, silicone, polyether, and mercaptan (polysulphide). Syringable impression material may be used for obtaining an occlusal record.
Stewardson D. A. Trends in indirect dentistry: 5. Impression materials and techniques. Dent Update 2005;32(7):374–6, 379–80, 382–4.