By simple definition, the reality principle—according to Freudian psychology and psychoanalysis—is...
...the ability of the mind to assess the reality of the external world, and to act upon it accordingly, as opposed to acting on the pleasure principle.
To clarify, the reality principle acts in the external world with an awareness of reality (being cognizant and modifying behavior based upon what is required at a specific moment by the environment at that time). The reality principle utilizes self-control.
For a deeper understanding, it is helpful to distinguish the "reality principle" from the "pleasure principle." The pleasure principle is not focused on reality, but upon perceptions that provide pleasure and avoid pain. The pleasure principle is what directs the id. The id, according to Freud, is the part of one's mind that sees to one's basic instincts: including "needs, wants, desires, and impulses." The id contains the libido, which is deemed to be that part of the mind that "is unresponsive to the demands of reality." It is impulsive, searching for instant gratification.
Conversely, the reality principle is associated with the ego, which is the organized, realistic part of the mind. It stands between the pleasure-seeking id, and the superego—which is "critical and moralizing." The reality principle is not looking for instant gratification; instead, this part of the mind has become well-developed enough to withstand the mind's desire for spontaneous satisfaction. While the pleasure principle guides one to get what he or she naturally wants (instantaneously), the reality principle is able to resist that immediate desire, holding out for enduring or lasting satisfaction. While Freud connected the pleasure principle to inexperience and immaturity, he associated the reality principle with education, responsibility and maturity.
Freud believed that the reality principle found a balance with which to satisfy the id (and the pleasure principle), manifesting itself in "socially appropriate ways."
This theory regards the reality to weigh the costs and benefits of an action before deciding to do it or abandoning it. For example, if you are in the store and see a really cute shirt, your first reaction could be oh I really want to try that on, even though it's in the middle store, but then once the reality principle kicks in you think of the consequences and how it would not be correct to do so.
A psychoanalytic concept, originally proposed by Sigmund Freud, that compels people to defer gratification when necessary due to the obstacles of reality. The reality principle is governed by the ego, which controls the instant-gratification mentality of the id.
The reality principle is the exact opposite of the pleasure principle (which seeks immediate gratification). One is looked upon as more mature when they are acting in accordance with reality. As children we seek pleasure and try to avoid things that cause us suffering or pain, however as we get older we realize that there are certain situations where it is better to endure the suffering/pain than to be out of control or inappropriate.