The Collaborative International Dictionary of English v.0.48:

Underwriter \Un"der*writ`er\, n.
   One who underwrites his name to the conditions of an
   insurance policy, especially of a marine policy; an insurer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underwriter
    n 1: a banker who deals chiefly in underwriting new securities
         [syn: investment banker, underwriter]
    2: an agent who sells insurance [syn: insurance broker,
       insurance agent, general agent, underwriter]
    3: a financial institution that sells insurance [syn: insurance
       company, insurance firm, insurer, insurance
       underwriter, underwriter]