Which term describes the improvised delivery of lines due to forgetting one's lines?

The term that describes the improvised delivery of lines due to forgetting one's lines is "ad lib." This practice occurs when actors spontaneously create dialogue or actions as a response to an unforeseen situation during a performance. It can arise from forgetting specific lines, needing to fill in gaps, or creatively responding to changes in the flow of a scene. Ad libbing allows performers to maintain the continuity of the performance and often requires quick thinking and creativity.

In contrast, an aside refers to a comment made by a character intended to be heard by the audience but not by other characters, which does not connect to improvisation or line forgetting. A bit part is a term for a small role in a play or film, focusing more on the size of the performance without implying any improvisation. A call refers to a prompt for actors to enter or start a scene and does not necessarily relate to line delivery in an improvised context.