Abba was a common word in the Aramaic and Hebrew languages, and meant ‘father’. It was a warm and informal term used in the everyday language of family life.
Jews of Old Testament times never used abba when addressing God, but Jesus used it when praying to his Father (Mark 14:36). The early Christians also addressed God as Abba; for, through Christ, God has adopted believers as his sons and made them joint heirs with Christ of his heavenly inheritance (Romans 8:15-17; Galatians 4:5-6; cf. Galatians 3:26; see ADOPTION).