:Natural class:

A group of sounds which are phonetically similar and which occur frequently as the input, output or environment of phonological rules. The following formal definition for natural class has been proposed: A class of segments which can be specified using fewer features than it would take to specify any member or any subset of the class. For example: voiceless plosives may be specified as [-son, -cont, -delrel, -voi]. A subset such as p t would need a specification which added [+ant]. p alone would need to add [+ant, -cor] to the specification. An arbitrary collection of segments, such as p ʒ m u does not have the formal property described above, and moreover, would not be expected to be treated as a class in the phonology of any language.