1) A doubting or questioning attitude or state of mind.
2a) The ancient school of Pyrrho of Elis that stressed the uncertainty of our beliefs
in order to oppose dogmatism.
2b) The doctrine that absolute knowledge is impossible, either in a particular
domain or in general.
2c) A methodology based on an assumption of doubt with the aim of acquiring
approximate or relative certainty.
3) Doubt or disbelief of religious tenets (TheFreeDictionary.com).
→ skeptic; → -ism.