Was 9/11 an Act of Terrorism?

No, it was not.

Terror is used to frighten a population in submitting to one's demands, and has been used in varying degrees of success; e.g., the imposition of socialism in the Soviet Union, the IRA demands for a united Ireland, etc.

Although we read much about what the 9/11 "terrorists" want, this has been all speculation. In fact, the only demands have been the ever-changing items presented in the occasional Osama Bin Laden videos, all of which are beyond contemplation, let alone implementation. And other "terrorist" attacks (such as Bali, Madrid, London, Tel Aviv, etc.) had no accompanying demands at all.

The supporters of these attacks, however, have clearly stated that they wished to murder people. They have not expressed any desire to "terrorize"; they have only expressed a desire to kill.

In short, 9/11 and associated acts are not terror, but are just plain mass murder as an end in itself.