Markedness, Marking, Markers
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Marking refers to changing form, and usually refers to the results of wider and regular systems of such change (such as inflection)
It need not be morphological; e.g. you can say that the use and choice of of articles (e.g. the, a) mark an object.
The term Marker usually refers to specific morphemes commonly used in morphemic marking. For example, in English the 's' as a suffix tends to mark plural form.
See also Modification.
Confusingly, marked forms can refer to markedness instead, and practice sees confusing uses of the terms.
Markedness refers to some linguistic behaviour as being irregular or unusual, often a case or small group of cases to be handled by an an exception an exception to a rule/description.
Unusual here is a broad term. For example, one aspect of institutionalized phrases is that they appear with markedly (i.e. unusually) high frequency.