I'd expand on Tom's explanation just a tiny bit. An allision is where you strike a fixed object. In the maritime usage you can have an allision by striking a dock, a bridge, or an anchored/moored ship. If the ship struck is underway but not making way, ie..stationary, to the best of my knowledge it's a collision and not an allision. I could be completely wrong on that finer point though and express only my current understanding of the definition.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.