A term, one of many to denote Muslims in the modern world. Others include "Islamic followers", "The Ummah", and of course "Muslims"
The differentiation of those supposedly involved in terrorist groups was initially done by muslims themselves to distance the true followers of the faith from those who were primarily obssessed with violence and little interested in the welfare of their fellow man, which is one of the primary concerns of Islam.
Lately though, the media has appropriated the term and used it to label any sort of protest movement against authorites that features muslims in its makeup. As such the term can be quite misleading. The distinction between terrorist (evil people who kill innocent people for purely political motives), and Muslim(nice people who care for their fellow man, worship God, and seek justice for all) should be clear enough to the intelligent observer, especially as terrorism is strictly forbidden in Islam. The usage of a term that mixes the two, like Islamist, may then be counterproductive, and quite confusing.