i.e. Magister officiorum.  Sophronius was a fellow student with Basil at Athens, and a friend of Gregory of Nazianzus.  He secured the favour of Valens, who was staying at Cæsarea in 365, by conveying him intelligence of the usurpation of Procopius at Constantinople.  (Amm. Marc. xxv. 9.)  On the circumstance which gave rise to this letter, cf. Greg. Naz., Ep. xviii.  Letters lxxvi., xcvi., clxxvii., clxxx., cxcii., and cclxxii. are addressed to the same correspondent, the last, as it will be seen, indicating a breach in their long friendship.