An aide is a person whose job is to help someone else do their work, especially someone in an important position. A teacher's aide helps the teacher manage the classroom and work with students. A nurse's aide assists nurses in hospitals by taking care of basic patient needs. In government, politicians often have aides who help them research issues, manage their schedules, and communicate with the public.

It specifically refers to an assistant in a professional setting. You usually wouldn't call your friend who helps you with homework an aide, but you would use it for someone whose actual job is to provide assistance.

Notice that aide sounds exactly like aid (which means help or assistance), but they're different words. Aid is the help itself; an aide is the person who provides it. A presidential aide might aid the president by preparing briefing documents. The Secret Service provides aid to protect the president, but they are not usually called aides because their role is specialized protection rather than general assistance.