MORPHEME is the minimal distinctive unit of grammar and the central concern of morphology. It is the smallest functioning unit in the composition of words. Words are the parts of words, such as suffixes, prefixes, and affixes. The smallest parts of expression associated with some meaning are called Morphemes. Morphemes are made up of the phoneme. It can also be defined as the minimal units of meaning out of which meaningful words are composed in various ways. A morpheme thus is a distinct linguistic form. It is a minimal unit of speech that is recurrent. It has a grammatical function. It is sentimentally different from other phonemically similar or identical linguistic forms and is not divisible analyzable into smaller forms. If we break or analyze a morpheme into constituents, it losses it identity and end up with a sequence of meaningless noise. Analyzing the morpheme leads us straight into the realm of phonology.
Morphemes may or may not have meaning, may or may not have a phonological representation (un) has a negative meaning in under or sun. In plural words like sheep, fish, we have two morphemes in each word, the first morpheme in each case has phonological representation but the second one has no phonological representation and is called Zero Morpheme. Morphologically the plural noun sheep in sheep i.e, that is to say, that word sheep plus a plural morpheme which is present in meaning but is not physically present in spelling or pronunciation.
Morphemes are commonly classified into three forms (Morphemes which can occur as separate words) and bound forms, thus unselfish consist of three morpheme un, self and ish, of which self is a free-form, un and ish are bound forms. A morpheme has been called a grammatical moneme by Morten. Another synonym for the morpheme is glosseme.