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A root (or root word) is a word that does not have a prefix in front of the word or a suffix at the end of the word. The root word is the primary lexical unit of a word, and of a word family (this root is then called the base word), which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents. Content words in nearly all languages contain, and may consist only of, root morphemes. However, sometimes the term "root" is also used to describe the word minus its inflectional endings, but with its lexical endings in place. For example, chatters has the inflectional root or lemma chatter, but the lexical root chat. Inflectional roots are often called stems, and a root in the stricter sense may be thought of as a monomorphemic stem.
The traditional definition allows roots to be either free morphemes or bound morphemes. Root morphemes are essential for affixation and compounds. However, in polysynthetic languages with very high levels of inflectional morphology, the term "root" is generally synonymous with "free morpheme". Many such languages have a very restricted number of morphemes that can stand alone as a word: Yup'ik, for instance, has no more than two thousand.
The root of a word is a unit of meaning (morpheme) and, as such, it is an abstraction, though it can usually be represented alphabetically as a word might be. For example, it can be said that the root of the English verb form running is run, or the root of the Spanish superlative adjective amplísimo is ampli-, since those words are clearly derived from the root forms by simple suffixes that do not alter the roots in any way. In particular, English has very little inflection and a tendency to have words that are identical to their roots. But more complicated inflection, as well as other processes, can obscure the root; for example, the root of mice is mouse (still a valid word), and the root of interrupt is, arguably, rupt, which is not a word in English and only appears in derivational forms (such as disrupt, corrupt, rupture, etc.). The root rupt can be written as if it were a word, but it is not.
This distinction between the word as a unit of speech and the root as a unit of meaning is even more important in the case of languages where roots have many different forms when used in actual words, as is the case in Semitic languages. In these, roots are formed by consonants alone, and speakers elaborate different words (belonging potentially to different parts of speech) from the root by inserting different vowels. For example, in Hebrew, the root gdl represents the idea of largeness, and from it we have gadol and gdola (masculine and feminine forms of the adjective "big"), gadal "he grew", higdil "he magnified" and magdelet "magnifier", along with many other words such as godel "size" and migdal "tower".
Secondary roots are roots with changes in them, producing a new word with a slightly different meaning. In English, a rough equivalent would be to see conductor as a secondary root formed from the root to conduct. In abjad languages, the most familiar of which are Arabic and Hebrew, in which families of secondary roots are fundamental to the language, secondary roots are created by changes in the roots' vowels, by adding or removing the long vowels a, i, u, e and o. (Notice that Arabic does not have the vowels e and o.) In addition, secondary roots can be created by prefixing (m-, t-), infixing (-t-), or suffixing (-i, and several others). There is no rule in these languages on how many secondary roots can be derived from a single root; some roots have few, but other roots have many, not all of which are necessarily in current use.
Consider the Arabic language:
Similar cases occur in Hebrew, for example Israeli Hebrew ?mqm 'locate', which derives from Biblical Hebrew ? måqom 'place', whose root is ?qwm 'stand'. A recent example introduced by the Academy of the Hebrew Language is midrúg 'rating', from ? midrág, whose root is ?drg 'grade'."
Consider also Rabbinic Hebrew ?trm 'donate, contribute' (Mishnah: T'rumoth 1:2: 'separate priestly dues'), which derives from Biblical Hebrew t'r?må 'contribution', whose root is ?rwm 'raise'; cf. Rabbinic Hebrew ?tr` 'sound the trummpet, blow the horn', from Biblical Hebrew t'r?`å 'shout, cry, loud sound, trumpet-call', in turn from ?rw`." and it describes the suffix.
In etymological reconstruction at the level of proto-languages, it is customary to reconstruct roots, which are assigned glosses, reflecting what is taken to be the common meaning shown by the words derived from this root.