Chopping people's heads off is pretty terrifying for the victims.
That is what MS-13 has been doing. That's terror.
Terrorism is loosely defined as the use of violence and threats to intimidate or coerce, especially for political purposes. Some definitions include the political requirement, others do not.
I don’t know anything personally about MS-13 but Wikipedia has a pretty good summary. They seem to be mostly criminal with no known ties to ISIS or AQ, but those lines can easily blur over time depending on who and how they recruit.
I don’t have any problem with Session using the terrorist label if it gives him additional tools to put these gangs down.