Segments are called "discrete" because they are separate and individual, such as consonants and vowels, and occur in a distinct temporal order. Other units, such as tone, stress, and sometimes secondary articulations such as nasalization, may coexist with multiple segments and cannot be discretely ordered with them. These elements are termed suprasegmental. It is not clear how the concept of segment applies to sign languages.
In phonology, there is a subfield of segmental phonology. It deals with the analysis of speech into phonemes (or segmental phonemes), which correspond fairly well to phonetic segments of the analysed speech.
When analyzing the inventory of segmental units in any given language, some segments will be found to be marginal, in the sense that they are only found in onomatopoeic words, loan words, or a very limited number of ordinary words, but not throughout the language. Marginal segments, especially in loan words, are often the source of new segments in the general inventory of a language. This appears to have been the case with English /ʒ/, which originally only occurred in French loans.