Source : Webster's Revised Unabridged Dictionary (1913)

Holla \Hol"la\, interj. [F. hola; ho ho + l[`a] there, fr. L.
   illac that way, there. Cf. {Hollo}.]
   Hollo.

Holla \Hol"la\, v. i. [imp. & p. p. {Hollaed}; p. pr. & vb. n.
   {Hollaing}.]
   See {Hollo}, v. i.

Source : WordNet®

holla
     n : a very loud utterance (like the sound of an animal); "his
         bellow filled the hallway" [syn: {bellow}, {bellowing}, {holler},
          {hollering}, {hollo}, {holloa}, {roar}, {roaring}, {yowl}]