A chemical that causes a reaction between other elements without being changed itself.
Season 3, Episode 20 - The Spaghetti Catalyst
Aired May 3, 2010
Leonard and Penny aren't talking to each other following their break-up, leaving Sheldon unsure how he to be friends with them both. Sheldon feels guilty about spending time with Penny, thinking it's a betrayal of his friendship with Leonard.
Season 4, Episode 14 - The Thespian Catalyst
Aired February 3, 2011
Sheldon is inundated with negative feedback when he teaches a class. To improve his communication skills, Sheldon gets acting lessons from Penny. Meanwhile, Raj has a series of romantic daydreams about his best friend's girl.
Season 5, Episode 4 - The Wiggly Finger Catalyst
Aired October 6, 2011
Penny introduces Raj to a girl he can actually communicate with - albeit with some help from Howard. Meanwhile, Sheldon frees up his mind by leaving trivial decisions to the roll of the dice.
Season 10, Episode 6 - The Fetal Kick Catalyst
Aired October 27, 2016
When Penny is invited to sign autographs at the Van Nuys Comic Con, she is surprised to find there are fans of "Serial Ape-ist". Amy convinces Sheldon to host friends in their new apartment, but Sheldon's comments to the assembled guests cause offense.
Meanwhile, Howard starts to freak out about being ready for the arrival of their baby, he makes a few impulse purchases without consulting Bernadette.