Just as other parts of speech, the concept of the adverb is a hybrid concept, combining structural and semantic criteria.

• By the structural criterion, an adverb is a word that may function as an adjunct.
• A functional concept of the adverb may be approximated by a procedure like this: Form a representative subset of one-word adjuncts by choosing one from each of the subclasses of
  • local modifier, e.g. ‘far’
  • temporal modifier, e.g. ‘always’
  • degree modifier, e.g. ‘completely’
  • manner modifier, e.g. ‘quickly’
  • modal modifier, e.g. ‘perhaps’.
  Ascertain their distribution and choose those words whose distribution is closest to the structural criterion. Then all words with the same distribution are adverbs.

The structural criterion is insufficient by itself because in most languages, words of other classes may function as adjuncts, too. It was traditionally sharpened by the additional requirement that the words in question do not inflect. If this is to work at all, it must be provided by some reserves:

• Certain morphological categories marked on adverbs do not count as inflection, but as derivation. This includes crucially
  • morphological comparison, as in German oft - öfter ‘often - more often’
  • expansion of adverbs by case suffixes, such as Finnish kau-kana (far-INESSIVE) - kau-as ‘far-LATIVE’ - kau-kaa (far-ABLATIVE).
• Words bearing markers of tense or aspect, as do certain Turkish and Kalaallisut words categorized as adverbs, involve a prior conversion into verbs.
• Modifiers which agree with their subject or absolutive in nominal categories, as in , are not adverbs, but predicative adjectives.

Some of these reserves need refinement in order not to be ad hoc.

The functional criterion characterizes a class that is semantically rather heterogeneous. As a consequence, those words differ somewhat in their distribution, and it becomes hard to determine a common denominator.

Thus, both the structural and the functional approach are insufficient by themselves. They must be combined so that they converge on a set of words.

Apart from this semantic heterogeneity, the class of adverbs is heterogeneous just like the other major classes (noun, adjective, verb) because it includes lexical elements like downwards and grammatical elements like there. Therefore, the distribution of lexical and of grammatical adverbs must be determined independently.