What an interesting question! I would have to say that the college environment incorporates all external factors related to attending college. This means that the environment is not only the physical space of attending college, such as dormitory rooms and the campus green, but the social and biological factors as well. The social environment of college is quite different from anything a student is likely to have experienced before or ever will experience again. College students are typically young adults exercising new-found independence and liberties. Lots of people try to "figure out" who they are during college by trying new hobbies, attending social events, and testing the boundaries they had previously set for themselves. Compared to primary schooling, the social bubble of college life is much larger, yet still somewhat enclosed.
In terms of the biology of college life, many changes may happen at this time! Personally, I've observed that many people come down with difficult illnesses like the flu during their first year of college. This is in part because of the sheer numbers of people who visit college campuses every day, but also influenced by the stress university life can put on a person's body. Students who have not yet learned to cook for themselves may choose to eat at the campus dining hall, which might offer everything from fast food to home-style meals to fruit smoothies. For most students, their typical diet before college and the diet during college are radically different. Many students have to compromise between time, effort, and nutrition when it comes to making decisions about what to eat.
The college environment isn't limited to just the physical grounds where the school sits, because a student who is enrolled in college may extend the environment to any place they go.