The Collaborative International Dictionary of English v.0.48:
Driftage \Drift"age\, n. 1. Deviation from a ship's course due to leeway. [1913 Webster] 2. Anything that drifts. [1913 Webster]WordNet (r) 3.0 (2006):
driftage n 1: the deviation (by a vessel or aircraft) from its intended course due to drifting