Define collision frequency.

Hint: From the name of collision frequency, it can be assumed that it is related with the collision between two species. The word frequency means the number of repeat occurrences of any particular event taking place per unit time.

Complete step by step solution:
Whenever a collision takes place between reactant molecules, a chemical reaction occurs. The number of collisions which are taking place per second per unit volume of the reaction mixture is referred to as collision frequency.
The other definition of collision frequency is “average rate at which two reacting species undergo collision in a particular system”.
The collision frequency is represented by the symbol Z. The value of collision frequency is always high. In case of binary collision, the value of collision frequency lies in the range of $${10^{25}} - {10^{28}}$$.
It can also be said that the number of collisions occurring per second per unit volume of the reacting species solution is referred to as collision frequency.
For example, a nitrogen $$\left( {{{\text{N}}_{\text{2}}}} \right)$$ molecule exists at a standard temperature that is equal to 298 Kelvin and a standard pressure that is equal to 1 bar consists of collision frequency equal to $$7 \times {10^9}\;{{\text{s}}^{ - 1}}$$(fixed value at standard conditions). This means within one second, one $${{\text{N}}_{\text{2}}}$$ molecule undergoes collision on average with 7 billion other molecules.
Additional Information: Collision theory is one of the factors which affect the rate of any specific chemical reaction. Let’s consider the following reaction where X and Y are reactants
$$X + Y \to {\text{Products}}$$
The rate of the above reaction is directly proportional to the collision frequency.
$${\text{Rate}} \propto {Z_{{\text{XY}}}}$$
In the above equation, $${Z_{{\text{XY}}}}$$ represents the collision frequency of X and Y reactants.

Note: Collision frequency is mainly utilized to express the number of collisions occurring per unit of time. Collision frequency is an important part of collision theory. Rate of any particular reaction depends upon the collision frequency. When collision frequency increases, the rate of chemical reaction increases as well.