Transition State: This is an unstable state of a system in which old bonds are partially broken and new ones are partially formed. It's like being halfway out the door when leaving your house - not fully inside anymore, but not completely outside either.
Catalyst: A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Like a good coach who helps players perform better without actually playing in the game themselves.
Reaction Mechanism: The step-by-step sequence of reactions by which the overall chemical change occurs. It's like following directions on Google Maps - each turn represents one step in getting to your final destination.