Linguistic: whether their mother tongue is Arabic; this definition covers more than 200 million people.
Genealogical: whether they can trace their ancestry back to the original inhabitants of the Arabian Peninsula.
The relative importance of these factors is estimated differently by different groups. Most people who consider themselves Arabs do so on the basis of the overlap of the political and linguistic definitions, but some members of groups which fulfill both criteria reject the identity on the basis of the genealogical definition. Not many people consider themselves Arab on the basis of the political definition without the linguistic one - thus, Kurds or Berbers usually identify themselves as non-Arab - but some do (for instance, some Berbers do consider themselves Arab.)
The genealogical definition was widely used in medieval times (Ibn Khaldun, for instance, does not use the word "Arab" to refer to "Arabized" peoples, but only to those of originally Arabian descent), but is usually no longer considered to be particularly significant.
In Islamic and Jewish tradition, Arabs are a Semitic people who trace their ancestry from Ishmael, a son of the ancient patriarch Abraham and Hagar. Medieval Arab genealogists divided the Arabs into two groups: the "original Arabs" of South Arabia, descending from Qahtan (identified with the biblical Joktan) and the "Arabized Arabs" (musta`ribah) of North Arabia, descending from Adnan, supposed to be a son of Ishmael. See Qahtanite.