This is a substance that forms during one of the middle steps of a reaction mechanism but does not appear in the overall balanced equation because it reacts in a later step.
This is the slowest step in a reaction mechanism. It's like waiting for your cake to bake - no matter how quickly you mixed your ingredients, you still have to wait for it to cook.
A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. It's like using an electric mixer instead of stirring by hand – it makes things go faster!