A zillion is an informal term that is used to refer to an extremely large number, often when the exact number is unknown or irrelevant. There are several synonyms for a zillion that one could use in their writing or conversation. One of the most common is "a gazillion," which is also used to refer to an unquantifiable amount. Other synonyms for a zillion include "umpteen," "countless," "innumerable," "myriad," "umpteen," and "multitude." Each of these words conveys a similar sentiment of a vast, uncountable number, and can be used interchangeably depending on the context and tone of the communication.