Root

The root of a word (a.k.a. the stem of a word) is the part of a word (morpheme) that carries the main meaning of the word.

In English, the root of a word is ALMOST always an independent word.

(This is not true for many other languages (e.g. Italian, Latin, etc.), where the root word MUST have some grammatical affix attached to the word. (These types of morphemes are called Bound Bases.))

Example
The words: all have the same root word: "teach". So "teach" is the root of all these words.
 * teach
 * teaching
 * teaches
 * teacher
 * taught

In English
In English affixes are often added to the roots of words to change a grammatical aspect of the word's meaning. (See Inflectional Affixes for more details.)