Final answer:
A scrim is a loosely woven fabric used in theatre productions that looks opaque when lit from the front and transparent when backlit.
Step-by-step explanation:
A loosely woven fabric that appears opaque when lit from one side and transparent when lit from the other is known as a scrim. This effect is made possible due to how the fabric interacts with light; when lit from the front, the light is diffused, making the scrim appear solid. Conversely, when the fabric is backlit, the threads become less visible, allowing objects behind it to be seen more clearly, creating a transparent effect. This property of scrim is often utilized in theatre productions to create mysterious or ethereal effects on stage.