Warning other boaters...

[nolatom]

Allision means one ship is stationary, while the other is in motion. Collision, (emphasis on "Co", meaning together) means both vessels were moving, both underway, in navigation.

And RoR means "Rules of the Road", also called the "COLREGs", meaning Collision Regulations in modern parlance.

[sailaway21]

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.