noun (n)

• something immaterial that stands in the way and must be circumvented or surmounted(noun.cognition)

  Example:
  Lack of imagination is an obstacle to one's advancement.
  The poverty of a district is an obstacle to good education.
  

  Synonym:
  obstruction
  

  source: wordnet30
• an obstruction that stands in the way (and must be removed or surmounted or circumvented)(noun.artifact)

  source: wordnet30
• That which stands in the way, or opposes; anything that hinders progress; a hindrance; an obstruction, physical or moral.(noun)

  source: webster1913