What is it called when an actor forgets his lines?

What is it called when an actor forgets his lines? The prompter (sometimes prompt) in a theatre is a person who prompts or cues actors when they forget their lines or neglect to move on the stage to where they are supposed to be situated.

The prompter (sometimes prompt) in a theatre is a person who prompts or cues actors when they forget their lines or neglect to move on the stage to where they are supposed to be situated.