trimer

The word "trimer" comes from roots meaning "three parts." It is a specialized scientific term for something made of three linked units. In many cases those units are identical, but they can also be closely related parts that join to form one whole.

Most commonly, you will encounter "trimer" in chemistry. It refers to a compound or molecular complex formed from three monomer units. Often this means three molecules of the same substance join together to make a larger structure. This process is called trimerization, and it can change the properties of the original substance.

In biology, the term is frequently used for protein trimers. Some proteins work properly only when three protein chains join in a specific shape. These can be homotrimers, with three identical chains, or heterotrimers, with three different chains. For example, some viral surface proteins form trimers, and scientists may describe an enzyme or receptor as active only in this three-part form.

Outside of scientific contexts, this word is rare. In general English, people usually say "trio" or "group of three" instead. "Trimer" is the precise choice when the idea is not just the number three, but three units joined into one structure, especially in chemistry and biology.