In psychology, rationalization is the process of constructing a logical justification for a decision that was originally arrived at through a different mental process.
This process can be in a range from fully conscious (e.g. to present an external defense against ridicule from others) to mostly subconscious (e.g. to create a block against internal feelings of guilt).
For an example, consider a person who bought one of the first home computers in 1980 primarily motivated by the excitement of playing with a computer. If he felt that his friends would not accept "having fun" as a sufficient reason for the purchase, he might have searched for other justifications and ended up telling them how much time it was going to save him in doing his taxes.
See also: cognitive dissonance
In economics, rationalization is an attempt to change a pre-existing ad-hoc workflow into one that is based on a set of published rules.
This is usually attempted to increase efficiency or flexibility.
Results tend to vary with the enthusiasm of the workers for the changes being made, with the skill in which management applies the rules to the work being done, and with the degree to which the rules fit the job at all.