Markedness is a specific kind of asymmetry relationship between elements of linguistic or conceptual structure. In a marked/unmarked relation, one term of an opposition is the broader, dominant one. The dominant default or minimum effort form is known as the 'unmarked' term and the other, secondary one is the 'marked' term. In other words, it is the characterization of a "normal" linguistic unit (i.e. the unmarked term) compared to the unit's possible "irregular" forms (i.e. the marked term).
In linguistics, markedness ranges over phonological, grammatical, and semantic oppositions, defining them in terms of 'marked' and 'unmarked' oppositions like honest (unmarked) vs. dishonest (marked). Marking may be purely semantic, or may be realized as extra morphology. The term derives from the 'marking' of a grammatical role with a suffix or other element, but has been extended to situations where there is no morphological distinction.
In the social sciences more broadly, markedness is used to distinguish two meanings of the same term, where one is common usage (unmarked sense) and the other is specialized to a cultural context (marked sense).
In psychology and machine learning markedness or deltap is the dual of informedness or deltap'. Empirically they measure the strength of associations, which match up remarkably with their interpretation as the probability that a condition marks a variable, resp. the probability that variable informs a condition or decision. The geometric mean of informedness and markedness, or equivalently of deltap and deltap', is Matthews correlation coefficient.
Read more about Markedness: Marked and Unmarked Word Pairs, Background in The Prague School, Jakobsonian Tradition, Cultural Markedness, Local Markedness and Markedness Reversals, Universals and Frequency, Diagnostics, Markedness in Generative Grammar