modal logic

from the GNU version of the Collaborative International Dictionary of English.

• noun A system of logic which studies how to combine propositions which include the concepts of necessity, possibility, and obligation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun logic Any formal system that attempts to deal with modalities, such as possibility and necessity, but also obligation and permission.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the logical study of necessity and possibility
• noun a system of logic whose formal properties resemble certain moral and epistemological concepts