Effective Collision
A collision that actually leads to reaction because it has both sufficient energy and correct orientation.
Unit
Dimensionless
In depth
Effective collisions are a tiny minority of all collisions, and the reaction rate is directly proportional to their frequency. Every factor that speeds a reaction — higher concentration, higher temperature, greater surface area, a catalyst — works by increasing the number of effective collisions per second.
Examples in the real world
Powdering a solid reactant increases surface area and therefore the number of effective collisions per second.