Terfenol-D is an alloy of terbium, dysprosium, and iron metals and has the largest room temperature magnetostriction of any known material.

The name Terfenol-D comes from the metallic elements; terbium (TER), iron (FE), Naval Ordnance Labs (NOL), and Dysprosium (-D). NOL developed and named the material, and is now known as Naval Surface Warfare Center - Carderock Division (NSWC-CD). The Navy developed Terfenol-D for higher power sonar that would also have greater bandwidth and greater reliability. ETREMA holds patents and licenses to many Terfenol-D applications, including the exclusive worldwide licenses to manufacture all types of Terfenol-D materials.

In technical terms, Terfenol-D is a solid-state transducer capable of converting very high energy levels from one form to another. In the case of electrical-to-mechanical conversion, the magnetostriction of the material generates strains 100 times greater than traditional magnetostrictives, and 2-5 times greater than traditional piezoceramics. The material has a high Curie temperature (380 C), which enables magnetostrictive performance greater than 1000 ppm from room temperature to 200 C. By adjusting the stoichiometry of the alloy, this temperature range can be extended down to cryogenic temperatures.