Collisional Frequency is the average rate in which two reactants collide for a given system and is used to express the average number of collisions per unit of time in a defined system.
Under certain conditions, the 2nd order kinetics can be well approximated as first order kinetics. These are pseudo first order. These are the reactions that appear to be second order in nature but are approximated as a first order reaction on close analysis. For example a second order reaction given by the equation : hydrolysis of ethyl acetate.