There are many, many, many definitions of terrorism. A study from back in the 80s, I think it was, showed there to be something like at least 109 definitions amongst policymakers, analysts, and academics. Surely there are even more today.
Usually, it boils down to something in the realm of nonstate actors directing politically motivated violence against civilians (though there's certainly debate about whether state actors can engage in terrorism as well as debate over other important issues and aspects related to terrorism, such as who the violence is directed at, etc.).
Interesting. I often wonder if some of these whackjobs just look for a "cause" to "justify" their actions. Can an act carried out by someone who may have visited a few websites but has no direct connection or backing from a terrorist sponsor really be called "terrorism" in the traditional sense, or is it just some heavily armed disaffected loser?
This has an overview of the situation & the person responsible: -
It was not a "terrorist" related incident.
RIP the poor souls that were run over by this total idiot.