Loyalty has just one definition, and that is "faithfulness or devotion to something or someone", any definition outside of this is a compromise to the truth.
The concept of loyalty has evolved over the years and is gradually taking a different form, where one stays faithful with a condition. If a condition is attached to loyalty, then it's no longer loyalty, but reciprocation of action. For example, Paul cheats if Paulina cheats. So it becomes a contract where one gets a reaction based on their actions.
However, Loyalty has no condition. Loyalty is love beyond what the other person is or can offer. Loyalty is doing what's right because you cherish it. Once the motive is wrong, then the entire equation becomes faulty. Loyalty is practiced in every area of life and for the purpose of this article, I'll talk about loyalty in a relationship.