Off book is a term that refers to someone who knows their lines or role in a performance without needing to refer to a script or notes. There are several synonyms for the term "off book" that can be used interchangeably in different contexts. "Memorized" is a common synonym, indicating that an individual has committed their part to memory. "Word-perfect" suggests precision in knowing the lines accurately. "Learned" implies that the lines have been studied and understood thoroughly. "Rehearsed" is another alternative that signifies the practice done to be able to perform without scripts. These terms share the same essence as "off book", signifying a level of familiarity and preparedness in delivering one's lines on stage or in a performance.