It seems like an obvious one, but it's easily missed in our fast paced world where a "white" lie sent by text is enticingly easy, and "ghosting" is actually a thing. Our personal integrity and the standard to which we hold ourselves is a measure of our own personal brand value. C. S. Lewis summed it up best with "Integrity is doing the right thing, even when no one is watching".
Integrity is doing the right thing even when no one would know you did the wrong thing. Making the brave choice even when there is an easier opt out to be gotten away with. Being in integrity means being in alignment with your core values even when no one knows the choice you are about to make but you.
When we are acting from integrity it means it's easier to accept the consequences of our decisions. When we do "the right thing" with honour, even if others do not approve or understand, we sleep well knowing we did what was in alignment with our own sense of rightness.
Don Miguel Ruiz, author of the great classic The Four Agreements details the trait of integrity as the number one agreement with self for personal freedom:
"1. Be Impeccable With Your Word
Speak with integrity. Say only what you mean. Avoid using the word to speak against yourself or to gossip about others. Use the power of your word in the direction of truth and love".
You cannot buy true integrity with money (reputation possibly, but not true integrity). And you cannot exchange it for money and you can't buy it back when it's gone (there are some notable falls from grace who would love that option I am sure. Think Lance Armstrong, Bill Cosby and Tiger Woods). Our integrity is of supreme value to us but it costs nothing. It is very hard to recover it once it has been lost, so it's important that we treasure and tend to it.
You do not need money or power or status to develop, cultivate and cherish your integrity. A life of integrity is a path open to everyone. In fact I think we are all pretty much naturally programmed to choose the path of integrity by default, which is why when we go against that which feels off. I believe that integrity is our natural inclination, it's more about choosing to deliberately veer off the path into dishonesty, or not.
I also like the alternate definition of integrity. As well as "the quality of being honest and having strong moral principles" it is also "the state of being whole and undivided". Which can be used to describe the state of, say, a building post earthquake "retaining its integrity" meaning it has come though structurally sound. That definition of being "whole and undivided" speaks also to our soul, in that if there is a decision that is making us feel divided and at odds with ourselves that it's likely we are considering compromising our integrity in some way. That feeling of internal division is a great spot check on our personal Integrit-o-meter.
When we live in integrity we are naturally happier. We don't have to think which version of the story we are relating to which person. We don't have that mild cloud of anxiety about "being found out". We sleep better. We don't panic when the phone rings or we bump into THAT person. Knowing what our word is worth is an all-together happier way to be in the world. We live more lightly when we are in integrity with ourselves.
Doing the right thing even if no one is watching is integrity. Because someone IS watching. Someone who's opinion is of paramount importance and will last a lifetime. You.