An attribute of something (a concept, an object, or a process) that manifests its perceived relative importance which, in turn, is linked to its conveyance of a truth, its scarcity, or its potential to fulfill a need, respectively.
For example, the value of the statement “All men are created equal, with unalienable rights for Life, Liberty and the pursuit of Happiness” stems from its conveyance of a truth. The value of gold stems from its relative rarity. The value of the skill-set of a surgeon or a plumber stems from their potential to fulfill a need.
Note that the attributed importance can be personal/individual (as in ‘sentimental value’) or collective/conventual (as in ‘the value of the American dollar’).