In most educational systems, undergraduate refers to the years of post-secondary education leading to a Bachelor's degree. Students of higher degrees are known as postgraduates (or often simply graduates).
In the United States, Lower-level Undergraduate refers to the first two years, while Upper-level Undergraduate refers to the last two years. Colleges, University Colleges, and Universities offer lower level undergraduate courses while Universities offer the upper-level courses. Most universities require a certain mixture of lower level and upper level courses to be completed for a Bachelor's Degree to be awarded. The Alma Mater Society (AMS) is usually the name of the Undergraduate student society of a university, while the Graduate Student Society (GSS) is the graduate counterpart.
In the United Kingdom, undergraduate refers to the first three or four years of study towards a Bachelor's degree.