(adj.) descriptive of an entity or situation where something once small and relatively insignificant grows exponentially at a swift pace, engulfing everything in its path. A more dramatic progression than the classic domino effect. The basic workings of a literal snowball effect can be illustrated by taking one's average baseball-sized snowball and dropping it down the side of a snowy hill. As it descends it gathers more snow and whatever leaves, sticks, etc. are in its way. The snowball accumulates not only size, but speed.
When Mortal Combat was first released, there were numerous instances of fans going to largely populated areas, like college campuses and shopping malls, and yelling "MORTAL COMBAT!!!" at the top of their lungs. Others withing earshot who understood this cry would do the same. Depending on just how many in the know heard and participated, created an amazing, echoing snowball effect.
the semi-scientific term snowball effect is used to discribe any object that gradually gains volume, such as when a snowball rolls down a hill. it is a term commonly used to describe people who have large weight gains.
the snowball effect can discribe young people who gain weight, mostly due to the natural cycle of growth. for example, while in middle school and just starting puberty, a preteen may be skinny and lanky. but as the reach their teen years, their body mass will gradually increase.
the snowball effect can also (although more harshly) apply to an adult who bulks up due to excessive eating or disease.
my girlfriend really succumbed to the snowball effect. when we were young kids in middle school and were just friends, she was really skinny and weighted under 90 lbs. while we were together in our high school years, she had a little meat on the bones, and was gradually getting bigger. by the time she was a sophomore in college, the snowball effect had caused her to weigh 145.