The phrase "ultimate cause" refers to the underlying reason or explanation for something. There are several synonyms that one could use to express this concept. For instance, the word "root cause" can be used to describe the fundamental reason for a problem or issue. Additionally, "primary cause" and "underlying cause" can be used interchangeably with "ultimate cause." "Proximate cause," on the other hand, relates to the immediate cause that triggers an event or action. Other synonyms that can be deployed depending on the context include "culprit," "origin," and "source." Regardless of the choice of phrasing, all these terms seek to identify the primary driving factor behind a particular phenomenon.