What does "junior" mean?

Additional senses

1. 2.Composed of juniors, whether younger or a lower standing; as, the junior class; of or pertaining to juniors or to a junior class. See Junior, n., 2.
2. 3.Belonging to a younger person, or an earlier time of life. Our first studies and junior endeavors. Sir T. Browne.
3. 4.A younger person. His junior she, by thirty years. Byron.
4. 5.Hence: One of a lower or later standing; specifically, in American colleges, one in the third year of his course, one in the fourth or final year being designated a senior; in some seminaries, one in the first year, in others, one in the second year, of a three years' course.