* A concentration within a major: Some universities might allow students to focus on a specific area *within* their major. This specialization is sometimes called a sub-major, concentration, or emphasis, but it's still part of the overall major. For example, a student majoring in Biology might concentrate in Marine Biology. This isn't a separate sub-major, but a focused track within the Biology major.
* A second major in progress: A student might informally refer to a second area of study they're pursuing, but haven't yet officially completed the requirements for a second major, as a "sub-major." This is just colloquial; they aren't officially declared as having a sub-major.
* A significant number of courses in a related field: A student might take many courses in a field closely related to their major, even if they don't meet the requirements for a minor. They might refer to this as a "sub-major," but again, it's not an official designation.
In short, while the term "sub-major" is sometimes used, it's generally not an official academic term. If you're interested in specializing within a major, look for options like "concentrations," "specializations," or "emphases" listed by the university's academic departments.