At what point is water considered "at a boil?" Lots of recipes call for waiting for the water to come "to a boil." However I have never been sure when that is. From my viewpoint it could be at three different points:


First start to see small air bubbles coming up from the bottom of the pan 
When the surface of the water is noticeably disturbed by the bubbles 
When it is a full on raging torrent of activity


Depending on the amount of water used, these three points could be several minutes apart from each other, which will affect the cooking time.
By definition, 1 is a simmer (once the bubbles form a steady stream), 2 is a boil, and 3 is a "roiling boil."