Abba (Ἀββά) is an Aramaic word meaning "father," transliterated into Greek (and then into our English Bibles). It was the word used by Jewish children for their father — intimate, direct, personal. It appears only three times in the New Testament (Mark 14:36; Romans 8:15; Galatians 4:6), always paired with the Greek translation ho Patēr ("the Father").
The word entered Christian usage through Jesus himself, who used it in Gethsemane — the most intimate crisis of his earthly life — to address God directly and tenderly.
The significance of Abba can scarcely be overstated. Jesus addressed God with this term as a constant expression of filial intimacy. Joachim Jeremias (though his conclusions have been nuanced by later scholars) argued that no Jew of Jesus' time addressed God as Abba in prayer — it was too presumptuous, too intimate. Whether or not that claim is fully accurate, Jesus' consistent use of Abba was striking and unique, expressing a relationship of perfect sonship.
What is breathtaking is that through the Spirit of adoption, this same word is now on the lips of every believer: "You have received the Spirit of adoption as sons, by whom we cry, 'Abba! Father!'" (Romans 8:15). The cry of Abba in the believer's heart is the Spirit's own witness that we are children of God — that the intimacy Jesus enjoyed with the Father has been extended to us through union with the Son.