In exchange for use of his facilities to build walls for his whale tank, Montgomery Scott gave him the formula for transparent aluminum. Scott brushed off concerns that this would result in a change of the timeline, asking, "How do we know he didn't invent the thing?" Nichols had recently quit smoking prior to Scott and McCoy's visit. (Star Trek IV: The Voyage Home)
Nichols was played by Alex Henteloff.
If history indeed credits Nichols with the invention of transparent aluminum, then Scott giving him the formula would be an example of a predestination paradox.
In the novel Star Trek IV: The Voyage Home, McCoy continues to argue against providing Nichols the formula after Scott asks the rhetorical question above, but Scott clarifies that history does, in fact, record that Dr. Nichols is the inventor of transparent aluminum, and hence Nichols was meant to receive the formula from him. The novelization gives Nichols' first name as "Marcus", and says that he prefers to have his friends call him "Mark." It adds that though he works for a plastics company, metallurgy is his first love.
According to the novel The Rise and Fall of Khan Noonien Singh, Volume Two, Nichols' first name is "Walter." Nichols patented transparent aluminum and passed it off as his own invention, eventually earning enough money to buy Plexicorp from its original owners. As a result of his work in the field of cryonics, he was recruited by Area 51 to assist in the construction of the DY-100 prototype vessel in March 1993.
The comic "Star Trek Special 1" gives his first name as "Brandon".
The All Our Yesterdays sourcebook gives his first name as "Jules".