Classifying speech units 
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.
Kinds of segment 
The segments of sign language are visual, such as hands, movements, face, and body. They occur in a distinct spatial and temporal order. The SignWriting script represents the spatial order of the segments with a spatial cluster of graphemes. Other notations for sign language use a temporal order that implies a spatial order.
In phonology, there is a subfield of segmental phonology that deals with the analysis of speech into phonemes (or segmental phonemes), which correspond fairly well to phonetic segments of the analysed speech.
Marginal segments 
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, interjections, 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.
Some phonemes cannot be easily analyzed as distinct segments, but rather belong to a syllable or even word. Such "suprasegmentals" include tone, stress, and prosody. In some languages, nasality or vowel harmony is suprasegmental.
- A Dictionary of Linguistics & Phonetics, David Crystal, 2003, pp. 408–409
- David Crystal, A Dictionary of Linguistics & Phonetics, Blackwell, 2003.
- Carlos Gussenhoven & Haike Jacobs, "Understanding Phonology", Hodder & Arnold, 1998. 2nd edition 2005.