Whatever an actor is wearing in performance is, by definition, a costume.
Costumes can be as costly and time-consuming to produce as scenery.
To the actor, costumes are the most important element of all. The costume, more than any other element, assists the actor in becoming the character. The restrictive fit of a period costume forces the actor to stand and move differently. Unlike the other design arts, there is really no important difference between costumes for the stage and screen. Costumes, whether elaborate period pieces or contemporary clothing seen on a soap opera have to be designed, acquired, and managed.