It would be very difficult to use words alone as a terrorist tactic. The words will need to be backed up by some sort of credible threat to qualify as terrorism. Terrorism attempts to change the minds of a given group of people by terrifying them. Words alone cannot cause terror.
To understand this, let us think of the example of a little child, perhaps four years old. Imagine if this child says that he will kill you. Will you be terrorized? It seems very unlikely. Thus, we can see that words alone cannot be a terrorist tactic.
However, if the words are backed by a credible threat of some sort, they can be terroristic. For example, imagine if Al-Qaeda were to announce that they were about to carry out a major attack on a US city. This would be much more credible because they have successfully carried out terrorist attacks around the world. In other words, it is not just their words, but their words backed by the possibility that their threat is real, that cause terror.
Words, then, cannot act alone. But if the words used by terrorists seem like plausible threats, they can be seen as acts of terrorism.