I don't think the answer is to create a new definition, though. The linguistic argument Oseleon made is in reverse: the accepted meanign includes property and military targets. That's what the word DOES MEAN now, by common consent. Better to come up with newer terms that describe a subset of that behavior. The important part of terrorism is not the target, but the result of the action. Ie, causing fear and terror.
This can be readily resolved by adding adjectives. But I don't think it's an issue requiring redefinition of a word that has an established, and useful, meaning.
Think about it, "violence" has a pretty darn broad meaning too. Some violence is legitimate: self defence, national defence, etc. But much of it is bad too. We have a large vocabulary describing subsets of violence, one of which is "terrorism." The only problem is that while terrorism describes a subset of violent behavior, it doesn't describe it in precise enough terms for every use we need. Thus, instead of limiting the term, I think we should create new terms with more limited application.