Source : Webster's Revised Unabridged Dictionary (1913)

Mourn \Mourn\, v. t.
   1. To grieve for; to lament; to deplore; to bemoan; to
      bewail.

            As if he mourned his rival's ill success. --Addison.

            And looking over the hills, I mourn The darling who
            shall not return.                     --Emerson.

   2. To utter in a mournful manner or voice.

            The lovelorn nightingale Nightly to thee her sad
            song mourneth well.                   --Milton.

   Syn: See {Deplore}.

Mourn \Mourn\, v. i. [imp. & p. p. {Mourned}; p. pr. & vb. n.
   {Mourning}.] [AS. murnan; akin to OS. mornian, OHG. mornen,
   Goth. ma['u]rnan.]
   1. To express or to feel grief or sorrow; to grieve; to be
      sorrowful; to lament; to be in a state of grief or
      sadness.

            Abraham came to mourn for Sarah, and to weep for
            her.                                  --Gen. xxiii.
                                                  2.

   2. To wear the customary garb of a mourner.

            We mourn in black; why mourn we not in blood?
                                                  --Shak.

            Grieve for an hour, perhaps, then mourn a year.
                                                  --Pope.

Source : WordNet®

mourn
     v 1: feel sadness; "She is mourning her dead child"
     2: observe the customs of mourning after the death of a loved
        one