ULTRA is a 5 letter word that starts with U.

1. \Ul"tra-\, a.
   A prefix from the Latin ultra beyond (see {Ulterior}), having
   in composition the signification beyond, on the other side,
   chiefly when joined with words expressing relations of place;
   as, ultramarine, ultramontane, ultramundane, ultratropical,
   etc. In other relations it has the sense of excessively,
   exceedingly, beyond what is common, natural, right, or
   proper; as, ultraconservative; ultrademocratic,
   ultradespotic, ultraliberal, ultraradical, etc.
   
   

2. \Ul"tra\, a. [See {Ultra-}.]
   Going beyond others, or beyond due limit; extreme; fanatical;
   uncompromising; as, an ultra reformer; ultra measures.
   
   

3. \Ul"tra\, n.
   One who advocates extreme measures; an ultraist; an
   extremist; a radical. --Brougham.