obstacle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something that stands in the way of or holds up progress.

from The Century Dictionary.

• noun That which opposes or stands in the way; something that obstructs progress; a hindrance or obstruction.
• noun Objection; opposition.
• noun Synonyms Difficulty, Obstacle, Obstruction, Impediment, check, barrier. A difficulty embarrasses, an obstacle stops us. We remove [or overcome] the one, we surmount the other. Generally the first expresses something arising from the nature and circumstances of the affair; the second something arising from a foreign cause. An obstruction blocks the passage, and is generally put in the way intentionally. An impediment literally clogs the feet and so may continue with one, hindering his progress, while a difficulty once overcome, an obstacle once surmounted, or an obstruction once broken down, leaves one free to go forward without hindrance.
• Obstinate; stubborn.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something that impedes, stands in the way of, or holds up progress

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun something immaterial that stands in the way and must be circumvented or surmounted
• noun an obstruction that stands in the way (and must be removed or surmounted or circumvented)