In English grammar the degree of comparison of an adjective or adverb describes the relational value of one thing with something in another clause of a sentence. An adjective may simply describe a quality, (the positive); it may compare the quality with that of another of its kind (comparative degree); and it may compare the quality with many or all others (superlative degree). In other languages it may describe a very large degree of a particular quality (in Semitic linguistics, called an elative).
The degree of comparison may be expressed morphologically, or syntactically. In English, for example, most monosyllabic and some disyllabic adjectives have morphological degrees of comparison: green (positive), greener (comparative), greenest (superlative); pretty, prettier, prettiest; while most polysyllabic adjectives use syntax: complex, more complex, most complex.
Traditional English grammar uses the comparative form when comparing exactly two things, and the superlative when comparing three or more, but in informal usage this may not hold.