Morpheme

One or more phonemes that combine to make a semantic unit that is not divisible into smaller semantic units.----

Again this is the minimal unit that is "meaningful" in a particular language, as determined by word pairs. For example, compare "athropy" and "dystrophy." Also, "euphoria" and dysphoria." Since "dys" distinguishes these pairs of words, "dys" is probably a morpheme.

It doesn't appear it can be further divided.----

Actually, there is not set definition of "morpheme' that I know. Once you consider "meaning"

in the definition, you wind up with a circular definition. And, you are combining structure with content. Some say that natives speakers, definition or not, know the morphemes in their own language. Others insist on defining this as the smallest "grammtical" unit in a language. For instance the /-s/ /-es/ /-en/ that pluralizes English nouns would be considered allomorphs of the "plural morpheme."