A transparent material as strong as metal, standard in 23rd-century construction, whose formula Scotty hands to a 20th-century engineer in Star Trek IV: The Voyage Home.
Transparent aluminium features in Star Trek IV: The Voyage Home. Stranded in the San Francisco of 1986, Kirk's crew need a tank to carry two humpback whales in the hold of their Klingon bird-of-prey, and have no money to pay for one.
Scotty and McCoy visit Plexicorp and offer its manager, Dr Nichols, the formula for transparent aluminium in exchange for large sheets of acrylic. Scotty first tries speaking to the computer through its mouse before typing the molecular structure on the keyboard. When McCoy worries about changing history, Scotty suggests that Nichols may have been the inventor all along.
The material is named again in The Next Generation episode In Theory as part of the Enterprise-D's windows, and it has come to stand for the windows and viewports of Starfleet ships in general.