Oxford Dictionaries defines terrorism as "the unlawful use of violence and intimidation, especially against civilians, in the pursuit of political aims."
US dictionary Merriam-Webster describes terrorism as "the systematic use of terror, especially as a means of coercion." There is, however, no commonly accepted definition. The term is often used in connection with unprovoked attacks and killings by an ideologically motivated group, like the so-called "Islamic State" (IS). But it is also sometimes used by governments to denounce opposition groups. This page collates DW content on terrorism.
The exchange follows threats from Iran's Revolutionary Guards to avenge the assault on a military parade. Iran-US ties have worsened after President Donald Trump renewed sanctions on Tehran over its nuclear program.