In linguistics, a segment is "any discrete unit that can be identified, either physically or auditorily, in the stream of speech". The term is most used in phonetics and phonology to refer to phones and phonemes, but can be applied for any minimum unit of a linear sequence meaningful to the given field of analysis, such as a mora or a syllable, a morpheme in morphology, or a chereme in sign language analysis.
Segments are called "discrete" because they are separate and individual, such as consonants and vowels, and occur in a distinct temporal order. Other contrastive elements of speech, such as prosody (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.
Segment (linguistics) Wikipedia
In phonetics, the smallest perceptible segment is a phone. 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.
The segmental phonemes of sign language (formally called "cheremes") are visual movements of hands, 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.
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 contrastive elements of speech cannot be easily analyzed as distinct segments but rather belong to a syllable or word. These elements are called suprasegmental, and include intonation and stress. In some languages nasality and vowel harmony are considered suprasegmental or prosodic by some phonologists.