A straight play is a term used to describe a form of theatrical production featuring spoken drama.

While music is one of Aristotle's six elements of theatre

Theatre

Theatre 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...

, a straight play does not usually include singing. However, some singing may be used for special characterisation

Characterisation

Characterization or characterisation is the art of creating characters for a narrative, including the process of conveying information about them. It may be employed in dramatic works of art or everyday conversation...

. (Usually, the term straight play does not include the Theatre of the Absurd

Theatre of the Absurd

The Theatre of the Absurd is a designation for particular plays of absurdist fiction, written by a number of primarily European playwrights in the late 1940s, 1950s, and 1960s, as well as to the style of theatre which has evolved from their work...

.) Straight play is commonly used to differentiate a non-musical a musical form.

Aristotle's six elements of theatre or play are known as plot, character, thought (theme), diction, music, and spectacle. (reference from this page)