A morph is a minimal form.
Colloquially, morphs are usually called "morphemes", but this term also has a range of technical uses. For example, one sometimes says that English has an abstract morpheme [PLURAL] that can be realized by -s (in student-s) or by -i (in alumn-i). But -s and -i are different forms, and hence different morphs (see Haspelmath 2020 for discussion). —— Sometimes morphs are said to be "phonetic realizations of (abstract) morphemes" (as in the SIL definition), or even that they belong to performance (language use) rather than the language system. But we clearly need a term for a minimal form (because roots, clitics and affixes are defined as kinds of minimal forms), so it is best to use "morph" in this sense. This seems to be what Bloomfield (1926: 155) had in mind with his term "morpheme"
"A minimum form is a morpheme; its meaning a sememe. Thus a morpheme is a recurrent (meaningful) form which cannot in turn be analyzed into smaller recurrent (meaningful) forms. Hence any unanalyzable word or formative is a morpheme." (Bloomfield 1926: 155) –– "Alternatively, morph can be used for the minimal sign (cf. Mel’čuk 1982: 63; Mel’čuk 2006: 388), in which case the unit “set of minimal signs with the same expression” has no name − a tolerable gap since this unit is of far less interest than the morpheme." (Mugdan 2015: 239)