A major (also known as a major concentration or concentration) is a term for the specific group of courses that give you a basic knowledge of a field of study, which is in addition to your core curriculum. A major is a term most commonly applied to a program of study leading to a bachelor's degree. The nursing or allied health school will define a framework for this specialized portion of your studies, including a certain number of required courses and a certain number of "elective" courses relevant to the major. The school will also define your general education or foundation education requirements. Some majors effectively define your full course of study while others allow you considerable latitude, both within your field of study and in their other courses.
Although many students choose their major before entering a university or college, many others will select it during their first or second year of a three- or four-year program. Typically schools do not allow you to officially declare your major until the end of your second year so that you experience a broad range of courses that will help you choose the most appropriate major.
Some schools actually require you to list a major choice on your application for admission, but this is rare. If you are interested in a major that requires a lot of classes, or classes that are limited to students in that major, you might have to declare earlier than usual (i.e., the fall semester of your second year). As well, for some majors you will need to take specific courses (pre-requisites) during your first and second year before you can even be considered eligible for upper level courses. Some nursing and allied health majors have limited enrolment so in fact you may even be required to apply to get the major you want, including attending an interview or writing an essay.
Community college programs generally do not have majors as such. Their 1- to 3-year diploma and certificate programs carry a designation, for instance diploma in Psychiatric Nursing, which is much more specific than, say, Bachelor of Nursing or Bachelor of Allied Health Sciences. Often, students in a community college program--sometimes all the students in the department--will take a common set of core foundational courses in the first year (or term, in the case of shorter programs) which then enables them to pursue a specialized area of study. But generally the field of study at the community college level is career-focused to the point where students do not take "elective" courses as they do for bachelor's degrees. Hence the designation of the diploma effectively stands in for the "major."