Morphology

Defition:

The branch of linguistics (and one of the major components of grammar) that studies word structures, especially in terms of morphemes. 

Observations:

• "The term 'morphology' has been taken over from biology where it is used to denote the study of the forms of plants and animals. . . . It was first used for linguistic purposes in 1859 by the German linguist August Schleicher (Salmon 2000), to refer to the study of the form of words. In present-day linguistics, the term 'morphology' refers to the study of the internal structure of words, and of the systematic form-meaning correspondences between words. . . .
  
  "The notion 'systematic' in the definition of morphology given above is important. For instance, we might observe a form difference and a corresponding meaning difference between the English noun ear and the verb hear. However, this pattern is not systematic: there are no similar word pairs, and we cannot form new English verbs by adding h- to a noun."
  (Geert E. Booij, The Grammar of Words: An Introduction to Linguistic Morphology, 2nd ed., Oxford Univ. Press, 2007)

• "For English, [morphology] means devising ways of describing the properties of such disparate items as a, horse, took, indescribable, washing machine, and antidisestablishmentarianism. A widely recognized approach divides the field into two domains: lexical or derivational morphology studies the way in which new items of vocabulary can be built up out of combinations of elements (as in the case of in-describ-able); inflectional morphology studies the ways words vary in their form in order to express a grammatical contrast (as in the case of horses, where the ending marks plurality)."
  (David Crystal, The Cambridge Encyclopedia of the English Language, 2nd ed. Cambridge Univ. Press, 2003)

• "The distinction between words and lexemes provides the basis for the division of morphology into two branches: inflectional morphology and lexical word-formation.
  
  "Inflectional morphology deals with the inflectional forms of various lexemes. It has something of the character of an appendix to the syntax, the major component of the grammar. Syntax tells us when a lexeme may or must carry a certain inflectional property, while inflectional morphology tells us what form it takes when it carries that inflectional property.
  
  "Lexical word-formation, by contrast, is related to the dictionary. It describes the processes by which new lexical bases are formed and the structure of complex lexical bases, those composed of more than one morphological element. The traditional term is simply 'word-formation.'"
  (Rodney Huddleston and Geoffrey K. Pullum, The Cambridge Grammar of the English Language. Cambridge Univ. Press, 2002)

 Morpheme

A meaningful linguistic unit consisting of a word (such as dog) or a word element (such as the -s at the end of dogs) that cannot be divided into smaller meaningful parts.

• "Morphemes can be divided into two general classes. Free morphemes are those which can stand alone as words of a language, whereas bound morphemes must be attached to other morphemes. Most roots in English are free morphemes (for example, dog, syntax, and to), although there are a few cases of roots (like -gruntle as in disgruntle) that must be combined with another bound morpheme in order to surface as an acceptable lexical item. . . .
  
  "Free morphemes can be further subdivided into content words and function words. Content words, as their name suggests, carry most of the content of a sentence. Function words generally perform some kind of grammatical role, carrying little meaning of their own. One circumstance in which the distinction between function words and content words is useful is when one is inclined to keep wordiness to a minimum; for example, when drafting a telegram, where every word costs money. In such a circumstance, one tends to leave out most of the function words (like to, that, and, there, some, and but), concentrating instead on content words to convey the gist of the message."
  (Steven Weisler and Slavoljub P. Milekic, Theory of Language. MIT Press, 1999)

• "A word like 'house' or 'dog' is called a free morpheme because it can occur in isolation and cannot be divided into smaller meaning units. . . . The word 'quickest' . . . is composed of two morphemes, one bound and one free. The word 'quick' is the free morpheme and carries the basic meaning of the word. The 'est' makes the word a superlative and is a bound morpheme because it cannot stand alone and be meaningful."
  (Donald G. Ellis, From Language to Communication. Lawrence Erlbaum, 1999)

• "There are two basic types of morphemes: unbound and bound. Unbound or free-standing morphemes are individual elements that can stand alone within a sentence, such as <cat>, <laugh>, <look>, and <box>. They are essentially what most of us call words. Bound morphemes are meaning-bearing units of language, such as prefixes and suffixes, that are attached to unbound morphemes. They cannot stand alone.
  
  "Their attachment modifies the unbound morphemes in such things as number or syntactic category. Adding the bound morpheme <s> to the unbound morpheme <cat> changes the noun's number; the addition of the <ed> to <laugh> changes tense. Similarly, the addition of <er> to <run> changes the verb to a noun."
  (Stephen Kucer and Cecilia Silva, Teaching the Dimensions of Literacy. Routledge, 2006)

• "Linguistics recognizes two classes of bound morphemes. The first class is called inflectional morphemes and their influence on a base word is predictable. Inflectional morphemes modify the grammatical class of words by signaling a change in number, person, gender, tense, and so on, but they do not shift the base form into another word class. When 'house' becomes 'houses,' it is still a noun even though you have added the plural morpheme 's.' . . .
  
  "Derivational morphemes constitute the second class of morphemes and they modify a word according to its lexical and grammatical class. They result in more profound changes on base words. The word 'style' is a noun, but if I make it 'stylish,' then it is an adjective. In English, derivational morphemes include suffixes (e.g., 'ish,' 'ous,' 'er,' 'y,' 'ate,' and 'able') and prefixes (e.g., 'un,' 'im,' 're,' and 'ex')."
  (Donald G. Ellis, From Language to Communication. Lawrence Erlbaum, 1999)