Trititanium

Trititanium is a type of material used in the construction of Orion scout ship hulls.

If sufficiently dense, trititanium can block the sensor probes of a Constitution Class starship. (TOS: "Journey to Babel")

According to Gene Roddenberry, trititanium is the material from which the hull of the USS Enterprise is made of in his novelization of Star Trek: The Motion Picture.

Trititanium may be the same substance as tritanium, although this has never been canonically established.