Literally, the habit of being given to enquiry.
The skeptic does not take things for granted. He may deny the existence of God, other minds than his own, a world of material objects behind what is immediately given to our senses, anything other than himself and his experiences (also see: solipsism), even his own mind as anything but a set of experiences (David Hume (1711-1776)), objective moral values, the possibility of getting any knowledge other than by the senses (also see empiricism), or by the senses (Plato sometimes), or of the past, or by the INDUCTIVE PRINCIPLE, or even by reason itself (Hume sometimes). Alternatively, the skeptic may simply doubt these things rather than deny them outright, and skepticism may be simply a methodological theory.
Among the Greeks, Plato's Academy came under the influence of skepticism for two centuries starting with ARCESILIUS (c.316-c.242 BC) and renewed by CARNEADES (c.214-c.l29 BC), and directed primarily against Stoicism.
A more extreme form of skepticism was Pyrrhonism, one of whose adherents, Sextus Empiricus (2nd century AD), is our main source of knowledge for ancient Skepticism.
Also see: private language argument
P Unger, Ignorance: A Case for Scepticism (1975)