"By then" is an expression that refers to a certain moment in time, indicating that something has happened or will happen before that point. There are several synonyms for this phrase that can be used to convey the same meaning, such as "by that time," "by the time," "until then," or "beforehand." Each of these phrases carries a different connotation depending on the context in which they are used. "By that time" and "by the time" are very similar in meaning and imply that something has happened within a certain period. "Until then" can convey a sense of anticipation or expectation, while "beforehand" implies that something has already occurred.