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]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "underwriter":
   accident insurance, actuary, annuity, approver, assurance,
   aviation insurance, bail bond, bailsman, bond, bondsman,
   business life insurance, casualty insurance,
   certificate of insurance, certifier, confirmer, cosignatory,
   cosigner, court bond, credit insurance, credit life insurance,
   deductible, endorser, endowment insurance,
   family maintenance policy, fidelity bond, fidelity insurance,
   flood insurance, fraternal insurance, godfather, godparent,
   government insurance, guarantee, guarantor, guaranty,
   health insurance, industrial life insurance, insurance,
   insurance agent, insurance broker, insurance company,
   insurance man, insurance policy, insurer, interinsurance,
   liability insurance, license bond, limited payment insurance,
   mainpernor, major medical insurance, malpractice insurance,
   marine insurance, mortgagor, mutual company, notary, notary public,
   ocean marine insurance, party, permit bond, policy, ratifier,
   robbery insurance, signatory, social security, sponsor,
   stock company, subscriber, surety, term insurance, theft insurance,
   upholder, warrantor



Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNDERWRITER, insurances. One who signs a policy of insurance, by which he
becomes an insurer.
     2. By this act he places himself as to his responsibility, in the place
of the insured. He may cause a re-insurance (q.v.) to be made for his
benefit; and it is his duty to act with good faith, and, without quibbling,
to pay all just demands against him for losses. Marsh. Ins. 45,