The intuition underlying the notion of markedness as used in contemporary linguistic theory is that, where a particular feature (in a broad, perhaps pretheoretic sense) has two (or more) values, one of these values is more usual, more expected, more natural than the other(s). This more usual value is called unmarked, the other value or values are called marked. (In the case of a feature with three or more values, it is conceivable that the different values could be ranged in order of markedness, from least to most marked.)

References

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1990 Markedness: the evaluative superstructure of language. State University of New York Press.

Bickerton, D.

1984 ‘The language bioprogram hypothesis’. The Behavioral and Brain Sciences 7: 173–221.  BoP

Chomsky, N. & M. Halle

1968 The sound pattern of English. Harper & Row.

Greenberg, J.H.

1966 Language universals with special reference to feature hierarchies. Mouton.

Hyams, N.

1986 Language acquisition and the theory of parameters. D. Reidel.  BoP

Jakobson, R.

1968 Child language aphasia and phonological universals. Mouton.

Trubetzkoy, N.S.

1968 Principles of phonology. Martinus Nijhoff.