Abba
Easton's Dictionary
This Syriac or Chaldee word is found three times in the New Testament (Mark 14:36; Rom. 8:15; Gal. 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.”
Smith's Dictionary
See Ab.
International Standard Bible Encyclopedia
ab'-a (abba, 'abba', Hebraic-Chaldaic, "Father"): In Jewish and old-Christian prayers, a name by which God was addressed, then in oriental churches a title of bishops and patriarchs. So Jesus addresses God in prayer (Mt 11:25,26, 26:39,42, Lu 10:21; 22:42; 23:34, Joh 11:41; 12:27; 17:24,25). In Mr 14:36; Ro 8:15, and Ga 4:6 ho pater, is appended even in direct address, in an emphatic sense. Servants were not permitted to use the appellation in addressing the head of the house. See Delitzsch on Ro 8:15; compare G. Dalman, Gram. des jud.-palast. Aramaisch, etc., section 40, c. 3.
J. E. Harry