'Goodbye' is the most common word used in English to acknowledge parting.

The word itself originated from 16th century usage of "God be with you." This original usage was identical to the common goodbye phrase still in use in arabic influenced languages like Urdu.

In Urdu, when you meet someone, you say 'salaam elaikum' (May God be with you), this is replied with 'wa alaikum asalaam' (May God also be with you), and when you part, you typically say 'Khuda Hafiz' (May God take care of you).
To understand how 'God be with you' metamorphosized to 'Goodbye', we should see the earlier forms of usage of the expression such as:
  • God be wy you,
  • god b'w'y,
  • godbwye,
  • god buy' ye, and
  • good-b'wy.
The first word of the expression changed from 'God' to 'good' most likely due to influence of such phrases as 'good day', perhaps after people no longer had a clear idea of the original sense of the expression.

A letter written in 1573 by Gabriel Harvey contains the first recorded use of the word 'goodbye': "To requite your gallonde (gallon) of godbwyes, I regive you a pottle of howdyes,"

Synonyms include: adieu, adios, arrivederci, auf wiedersehen, au revoir, bye, bye-bye, Ciao, cheerio, goodby, good-bye, goodbye, good day, Later, sayonara, so long, tata