The Collaborative International Dictionary of English v.0.48:

Junior \Jun"ior\, n.
   1. A younger person.
      [1913 Webster]

            His junior she, by thirty years.      --Byron.
      [1913 Webster]

   2. Hence: One of a lower or later standing; specifically, in
      American colleges and four-year high schools, 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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Junior \Jun"ior\ (j[=u]n"y[~e]r; 277), a. [L. contr. fr.
   juvenior, compar. of juvenis young. See Juvenile.]
   [1913 Webster]
   1. Less advanced in age than another; younger. Abbreviated
      Jr.
      [1913 Webster +PJC]

   Note: Junior is applied to distinguish the younger of two
         persons bearing the same name in the same family, and
         is opposed to senior or elder. Commonly applied to a
         son who has the same Christian name as his father.
         [1913 Webster]

   2. Lower in standing or in rank, or having entered later into
      a position or office; as, a junior partner; junior
      counsel; junior captain; the junior Senator from New York.
      [1913 Webster]

   3. Composed of juniors, whether younger or a lower standing;
      as, the junior class; the junior baseball league; of or
      pertaining to juniors or to a junior class. See Junior,
      n., 2.
      [1913 Webster]

   4. Belonging to a younger person, or an earlier time of life.
      [1913 Webster]

            Our first studies and junior endeavors. --Sir T.
                                                  Browne.
      [1913 Webster]

   5. of or relating to the third year of a four-year term; --
      used of the third or next to final year in a U. S. high
      school or college. See junior[2], n..

   Syn: third-year.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

junior
    adj 1: younger; lower in rank; shorter in length of tenure or
           service [ant: senior]
    2: used of the third or next to final year in United States high
       school or college; "the junior class"; "a third-year student"
       [syn: junior(a), third-year, next-to-last]
    3: including or intended for youthful persons; "a junior sports
       league"; "junior fashions"
    n 1: term of address for a disrespectful and annoying male;
         "look here, junior, it's none of your business"
    2: a third-year undergraduate
    3: the younger of two persons; "she is two years my junior"
    4: a son who has the same first name as his father [syn:
       Junior, Jr, Jnr]