An undergraduate is a university student who is still working toward their first degree — a bachelor's degree. Once they finish and go on to a master's or a doctorate, they are a graduate student (or in British English, a postgraduate). So the word marks a stage, not an age: a fifty-year-old starting a bachelor's degree is an undergraduate too. The word works both as a noun ('she is an undergraduate') and as an adjective before another noun ('undergraduate courses', 'undergraduate students', 'my undergraduate years'). In everyday speech people shorten it to 'undergrad', which is very common in American English and in student talk everywhere. Two things often confuse learners. First, the word describes someone who has NOT yet graduated — the under- here means 'below that level', not 'under a graduate' in any physical sense. Second, in ordinary conversation English speakers usually do not use it at all; they just say 'I'm a student' or 'I'm at university'. 'Undergraduate' comes out when the difference from graduate study actually matters — on forms, in admissions offices, in course catalogues, in job ads asking for 'an undergraduate degree'. In that last phrase it simply means a bachelor's degree.