The term spammable refers to skills or abilities that can be used over and over again, repeatedly. Such abilities do not have high resource costs, or cooldowns, or else there are ways to overcome such limits. For instance, a character with very high resource regeneration might be said to have made even a high cost skill spammable.
Most spammable skills in Diablo III are low or mid-level, and by definition, not as powerful as the higher level skills that are not spammable by dint of their long cooldowns.
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