Yes. There are many definitions of terrorism, but most definitions have four elements in common:
Violence;
Perpetrated by an individual or non-governmental group
Political, social, or religious motivations;
Intimidating a wider audience than the immediate victims.
By this definition, the massacre in Charleston, S.C. Wednesday was clearly a terrorist act. The violence is evident in the death toll of nine people. The perpetrator apparently was not soldier or official acting on behalf of a government, which would make it a different category of violence.
Most important to me is the author's second question, which highlights the moral dimension of using such a politically charged word:
2. Why does it matter if we call it “terrorism”?
The word “terrorism” matters, and not only for theoretical or academic reasons. Terrorism has causes and effects that are distinct from the causes and effects of, say, common crime. If policymakers want to think about how to address terrorism, in addition to typical crime-fighting tactics, how incidents are understood matter...
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Finally, it is important to use the term “terrorism” when we see it in order to be consistent. Terrorism is already used without hesitation for many non-white – especially Muslim – actors who carry out violence consistent with the definition outlined above. Few media sources use the term for violent actors motivated by, for example, white supremacy or anti-government rage. To avoid the term becoming simply an insult, or worse a racist insult, it should be used whenever the basic criteria apply, or not at all.