Plexiglass, also known as "Plexiglas" or by its chemical name, polymethyl methacrylate, was a strong, transparent polymer plastic used for viewing barriers like windows and aquarium tanks. It was the immediate predecessor to transparent aluminum.
In the Earth city of San Francisco, the company known as Plexicorp was well-known for its output of plexiglass. In 1986, Montgomery Scott and Leonard McCoy from the 23rd century gave Plexicorp manager Dr. Nichols the plans for transparent aluminum in exchange for a pane of the plexiglass, which they required to build a whale tank capable of holding two humpback whales.
Although there was some doubt, the plexiglass tank was able to withhold the pressure of the tons of water transported to the ship along with the whales. With the whales secure in the plexiglass tank, the Bird-of-Prey returned to the future to release them into Earth's ocean, thus giving them a chance to repopulate their species. (Star Trek IV: The Voyage Home)