What does the word harpagmos mean in Philippians 2:6?

MOUNCE renders it: who, although he was in the form of God, did not regard equality with God a thing to be grasped, Westcott and Hort / [NA27 variants] ὃς ἐν μορφῇ θεοῦ ὑπάρχων οὐχ ἁρπαγμὸν ἡγήσατο τὸ εἶναι ἴσα θεῷ, Whereas ...