From the Greek for "inquirer", it's someone who doubts the claims of philosophical knowledge
Some skeptics have challenged knowledge claims so that religious ones 
.... to a negative dogmatic conclusion from its doubts, Pyrrhonian skepticism 
made... [DOC] [TLE] Socrates | Internet Encyclopedia of Philosophy [PAR] Presocratic Philosophy and the Sophists; Socratic Themes in Plato's Apology .... 
that were officially sanctioned by the city and its officials marked ancient Greek 
religion.