it is used to express either hello or good bye. In french we say 'SALUT'

Let us begin with a history of the word and how it came into English from Italian:

"Ciao first appears in English in 1929 in Hemingway's "A Farewell to Arms," which is set in northeast Italy during World War I. It is likely that this is where Hemingway learned the word, for ciau in Venetian dialect means “servant, slave,” and, as a casual greeting, “I am your servant.” Ciau corresponds to standard Italian schiavo; both words come from Medieval Latin sclavus, “slave.” A similar development took place with servus, the Classical Latin word for “slave,” in southern Germany, Austria, Hungary, and Poland, where servus is used as a casual greeting like ciao. At the opposite end of the world, in Southeast Asia, one even sees words meaning “slave” or “your slave” that have developed into pronouns of the first person, again to indicate respect and humility" (The American Heritage Dictionary).

If one were to find an exact equivalent for it in Arabic, the closest would be:

خادمكم
في خدمتكم
سـيدي

. . . and similar expressions that show a sentiment of polite (but not literal) submission.

If, however, we simply want a word that expresses a multi-functional greeting (for both hello and good-bye) then the best term would be

سلام، سلام عليكم، السلام عليكم



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Note added at 2003-03-22 13:20:17 (GMT)
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The word is pronounced CHOW (rhymes with COW).

جاو (كلمة ايطالية) تستعمل عند الالتقاء بالاصدقاء وعند الافتراق