I think I'm safe in saying that just about everything happens because of something else ("...for every action there is an opposite and equal reaction"). Take a look at the picture on the right. Without the ball the dominoes would simply stand there. But because the ball happened to move in just the right direction at just the right speed and bumped into the first domino, the entire line of dominoes reacts. The ball was the action--the catalyst--that created a reaction.
Catalysts are generally encountered in and associated with chemistry. According to dictionary.com, a catalyst is a substance that causes or accelerates a chemical reaction without itself being affected. In literature it is something that causes activity between two or more persons or forces without itself being affected; a person or thing that precipitates an event or change. It can be a person whose talk, enthusiasm, or energy causes others to be more friendly, enthusiastic, or energetic. The catalyst can be thought of as an instigator whose job it is to keep the story moving using action to further accelerate conflict. These guys love to create disturbances in the story’s flow, and they can sometimes be quite annoying!
The catalyst in literature is usually a character, but it does not have to be. In The Perfect Storm the weather is the catalyst that creates the premise for the entire outcome. In J.K. Rowling's Harry Potter series multiple catalysts exist, both characters and non-characters, that motivate actions: Worm Tail, Sirius Black, Luna Lovegood, Rita Skeeter, George and Fred Weasley (the twins), magic and spells, to name a few. Think about these characters' and non-characters' roles. They are definitely more than merely flat characters (as discussed in last week's blog), as they actually move the story along and are a part of the plot. They appear over and over throughout the entire series, though they are not the main characters.