Abba

A Syriac word signifying father. When the Jews came to speak Greek, this word may have been retained from their ancient language, as being easier to pronounce, especially for children, than the Greek pater. It expressed the peculiar tenderness, familiarity, and confidence of the love between parent and child, Mark 14:36; Romans 8:15; Galatians 4:6.

Source: ATS Bible Dictionary
Abba

This Syriac or Chaldee word is found three times in the New Testament (Mark 14:36; Romans 8:15; Galatians 4:6), and in each case is followed by its Greek equivalent, which is translated "father." It is a term expressing warm affection and filial confidence. It has no perfect equivalent in our language. It has passed into European languages as an ecclesiastical term, "abbot."

Source: Easton's Bible Dictionary
Abba

AB'BA, n. In the Chaldee and Syriac, a father, and figuratively a superior. appen.

In the Syriac, Coptic and Ethiopic churches, it is a title given to the Bishops, and the Bishops bestow the title, by way of distinction, on the Bishop of Alexandria. Hence the title Baba, or Papa, Pope or great father, which the Bishop of Alexandria bore, before the Bishop of Rome.

Source: King James Dictionary