In warfare, a theater, is an area or place in which important military events occur or are progressing. A theater can include the entirety of the air, land, and sea area that is or that may potentially become involved in war operations.

In his book On War, Carl von Clausewitz defines the term as one that:

"Denotes properly such a portion of the space over which war prevails as has its boundaries protected, and thus possesses a kind of independence. This protection may consist in fortresses, or important natural obstacles presented by the country, or even in its being separated by a considerable distance from the rest of the space embraced in the war. Such a portion is not a mere piece of the whole, but a small whole complete in itself; and consequently it is more or less in such a condition that changes which take place at other points in the seat of war have only an indirect and no direct influence upon it. To give an adequate idea of this, we may suppose that on this portion an advance is made, whilst in another quarter a retreat is taking place, or that upon the one an army is acting defensively, whilst an offensive is being carried on upon the other. Such a clearly defined idea as this is not capable of universal application; it is here used merely to indicate the line of distinction."

Theatre (in American English usually theater) is a collaborative form of fine art that uses live performers to present the experience of a real or imagined event before a live audience in a specific place. The performers may communicate this experience to the audience through combinations of gesture, speech, song, music or dance. Elements of design and stagecraft are used to enhance the physicality, presence and immediacy of the experience. The specific place of the performance is also named by the word "theatre" as derived from the Ancient Greek θέατρον (théatron, “a place for viewing”) and θεάομαι (theáomai, “to see", "to watch", "to observe”).

Modern Western theatre derives in large measure from ancient Greek drama, from which it borrows technical terminology, classification into genres, and many of its themes, stock characters, and plot elements. Theatre scholar Patrice Pavis defines theatricality, theatrical language, stage writing, and the specificity of theatre as synonymous expressions that differentiate theatre from the other performing arts, literature, and the arts in general.