CONFLUENT is a 9 letter word that starts with C.

1. \Con"flu*ent\, a. [L. confluens, -entis, p. pr. of
   confluere, -fluxum; con- + fluere to flow. See {Fluent}.]
   1. Flowing together; meeting in their course; running one
      into another.
   
            These confluent steams make some great river's head.
                                                  --Blackmore.
   
   2. (Bot.) Blended into one; growing together, so as to
      obliterate all distinction.
   
   3. (Med.)
      (a) Running together or uniting, as pimples or pustules.
      (b) Characterized by having the pustules, etc., run
          together or unite, so as to cover the surface; as,
          confluent smallpox. --Dunglison.
   
   

2. \Con"flu*ent\, n.
   1. A small steam which flows into a large one.
   
   2. The place of meeting of steams, currents, etc. [Obs.]
      --Holland.