The term phonology can be used in a wide sense, to refer to the study of all aspects of the sounds of human language and of individual languages, and in a narrow sense, to refer to functional aspects of language and languages. In the narrow sense it is in contrast to phonetics, which is the study of the language sounds themselves. (That is a very brief and inadequate definition of 'phonetics' - if you want a fuller picture, follow the link!)
Phonology as a discipline can be studied synchronically, as a
subject of general linguistics, or diachronically, as a subject
of historical linguistics.
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