Activation energy ($E_a$) is the minimum energy required for colliding particles to form an activated complex (also known as the transition state) and proceed to products.

An elementary step is a single molecular event in a reaction mechanism where reactants are converted into products or intermediates.

An intermediate is a species that is formed in one elementary step and consumed in another. It is not present in the overall reaction equation because it does not appear in the final products or reactants.

The molecularity of an elementary step refers to the number of reacting particles (atoms, ions, or molecules) that must collide simultaneously to drive a chemical change.

The slowest step in the mechanism is called the rate-determining step (RDS).

A transition state represents the highest-energy arrangement of atoms during an elementary step. It is the point where bonds are partially broken and formed, and the system is at the peak of its energy profile.