Salient is an adjective meaning extremely necessary or stands out from the rest. It is also a synonym for jumping and rocketing upwards. Lastly, it is sometimes a noun used for an object that juts out from the rest of the environment, especially if it is part of military defenses.
A common phrase is to call a good or valid point in an argument a salient point.
Salience is the noun form of the condition of being salient. It may also be used to call something a highlight, or a definitive attribute.
Saliency is a synonym for salience and there is no difference between the two. Salience is more popular, being used slightly more than three times for each saliency. The exception is in the business world, where saliency is used as a term for brand awareness; in other words, the degree to which the brand sets itself apart from competition.