Terrorism is, in a sense, very hard to define. That is because an act that one person sees as terrorism may be seen as legitimate by another person. For example, the link below gives the following as one definition of terrorism:
…the illegal use of force or violence to accomplish a political or social goal.
But this definition is not perfect because it does not differentiate between the use of force or violence to accomplish a legitimate goal and the use of force or violence to accomplish an illegitimate goal. Think about it this way: the violence used by the American colonists against their British government was clearly illegal and was clearly being used to accomplish a political goal. Why, then, was it not terrorism?
Most people would probably define terrorism, then, as violent actions taken against innocent victims for political or social purposes. This is a better definition, but it still means that terrorism is in the eye of the beholder. Each person has to decide for themselves which victims are innocent and which are not.
As to terrorism in the 21st century, it is likely to be much the same as it always has been. Terrorists will continue to try to kill civilians as a way of terrorizing a group that they do not like. They will use violence to try to pressure governments to change their policies. Terrorism in the 21st century may be more lethal than in previous times if terrorists have more weaponry, but the basic ideas and purposes of terrorism will not change.