1. Of or relating to a musical mode; especially written in an ecclesiastical mode.
2. A description of propositions involving necessity or probability, or those relating to knowledge, belief, and obligation.
3. In grammar, of or concerning the mood of a verb.

A set of verb forms or inflections used to indicate the speaker's attitude toward the factuality or likelihood of the action or condition being expressed.

In English the indicative mood is used to make factual statements, the subjunctive mood is used to indicate doubt or unlikelihood, and the imperative mood is used to express a command.

4. Etymology: a term in logic, from Middle French modal, from Middle Latin modalis, "of or pertaining to a mode"; from Latin modus, "measure, manner, mode".