There are several synonyms for the term "second mate" depending on the industry or context. In the maritime industry, the term "second officer" is often used interchangeably. In aviation, a "co-pilot" or "first officer" is the equivalent of a second mate on a ship. In the military, the term "executive officer" is commonly used. Other terms that may be considered synonyms for a second mate include "assistant navigator," "deck officer," or "junior officer." It's important to pay attention to the specific industry or context in which the synonym is being used as some may have slightly different roles or responsibilities.