Snowballing is a concept where once a hero gets enough momentum, they become much more stronger (relative to other heroes at that stage) due to that momentum. The general definition of snowballing can be found here (http://en.wikipedia.org/wiki/Snowball_effect)

In DotA2 snowballing can be used in many different contexts. When a hero is said to be good at snowballing this means that when the hero gains enough of an advantage (usually in early or mid game) this allows them them to completely dominate the enemy team. When a team is said to snowball, this implies that due to a teams certain composition, when a team acquired an advantage they are able to keep (and increase) this advantage.

Note that heroes that snowball are not the same thing as Carries. Carries are heroes that scale very well with items (and hence they become quite strong late game when they acquire these items). On the other hand heroes that snowball are ones that are able to maintain and increase their mom…
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