If what Saudi Arabia, with American assistance, is doing to Yemen isn’t terrorism, what is? From Caitlin Johnstone at caitlinjohnstone.com:

The Trump administration is reportedly close to moving the Houthi rebels in Yemen onto its official list of designated terrorist organizations with the goal of choking them off from money and resources. The head of the UN’s World Food Program along with many other experts caution that this designation will prolong the horrific war which has claimed over a quarter million lives and create an impenetrable barrier of red tape stopping humanitarian aid from getting to the Yemeni people.
The United Nations conservatively estimates that some 233,000 Yemenis have been killed in the war between the Houthis and the US-backed Saudi-led coalition, mostly from what it calls “indirect causes”. Those indirect causes would be disease and starvation resulting from what UN Secretary-General Antonio Guterres calls “the worst famine the world has seen for decades”.
When people hear the word “famine” they usually think of mass hunger caused by droughts or other naturally occurring phenomena, but in reality the starvation deaths we are seeing in Yemen (a huge percentage of which are children under the age of five) are caused by something that is no more natural than the starvation deaths you’d see in a medieval siege. They are the result of the Saudi coalition’s use of blockades and its deliberate targeting of farms, fishing boats, marketplaces, food storage sites, and cholera treatment centers with airstrikes aimed at making the Houthi-controlled parts of Yemen so weak and miserable that they break.
In other words, the US and its allies have been helping Saudi Arabia deliberately kill children and other civilians on mass scale in order to achieve a political goal. Which would of course be a perfect example of any standard definition of terrorism.