'Doubter' definitions:

Definition of 'doubter'

From: WordNet
noun
Someone who habitually doubts accepted beliefs [syn: skeptic, sceptic, doubter]
noun
Someone who is doubtful or noncommittal about something [syn: agnostic, doubter]

Definition of 'Doubter'

From: GCIDE
  • Doubter \Doubt"er\, n. One who doubts; one whose opinion is unsettled; one who scruples. [1913 Webster]