[…] the notion of markedness was first introduced to characterize certain properties of phonological and morphological oppositions. Consider, first of all, the following simple example: (48) singular: ‘book’, plural: ‘book’-‘s’ Note that the plural form is characterized by the explicit presence of a plural ending, where the singular is characterized by the absence of any ending. This presence vs. absence of a formal marker corresponds, in a sense, to a semantic difference: the plural form is used when explicit reference is to be made to more than one entity of the type “book”; but the use of the singular form is not restricted to indicating just one entity of the type “book”, since in those cases in which the number of “book-entities” is irrelevant (i.e. may be one or more) we also find the form “book”, as in “book market”,“book production”,“book collector”,“book-ish”, etc. Rather than to say that “book” signals “singular”, it would thus be more appropriate to say that it signals “non-plural”, in the sense that it does not explicitly inform us about plurality, although it is not incompatible with a plural interpretation. This opposition can now be represented as follows: (49) ‘book’ ‘books’ markedness: unmarked marked form: absence of suffix presence of suffix meaning: “non-plural” “plural” […].
- Dik (1989), a pag.40

A construction type is more marked to the extent that it is less expectable, and therefore commands more attention when it occurs. In general, the less frequent, the more rare a linguistic item is, the higher its markedness value.
- Dik (1989), a pag.38

The term was originally introduced by Jakobson [Jakobson, 1936, “Beitrag zur allgemeinen Kasuslehre”, Travaux du Cercle Linguistique de Prague 6: 240-288] and Trubetzkoy [Trubetzkoy, 1939, Grundzϋge der Phonologie, Prague, Travaux du Cercle Linguistique de Prague 7] to indicate certain relationships within phonological and morphological “oppositions”. […] More recently, the term has also been used to indicate “marked” and “unmarked” construction types, both within and across languages.
- Dik (1989), a pag.38