Prelap is a screenwriting term that describes when the audience is introduced to a nonvisual aspect of a scene, such as a sound or line of dialogue, before the cut to the scene. As an example:

In this example, Adrian's voice precedes the scene out in the woods. The "V.O." means "Voice Over" and the "PRELAP" indicates that Adrian's dialogue should be heard before the next scene begins. Adrian, in this example, might not even be in the scene the other characters are in when the prelap occurs. ("O.S." or "Off Screen" would not be appropriate as the term should only be used for characters unseen but on set.)

Prelaps can be of sound or dialogue, or anything non-visual, since a visual would indicate a direct cut to a new scene.