obstacle

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One that opposes, stands in the way of, or holds up progress.

Century Dictionary and Cyclopedia

1. n. That which opposes or stands in the way; something that obstructs progress; a hindrance or obstruction.
2. n. Objection; opposition.
3. n. 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.
4. Obstinate; stubborn.

Wiktionary

1. n. Something that impedes, stands in the way of, or holds up progress

GNU Webster's 1913

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

WordNet 3.0

1. n. something immaterial that stands in the way and must be circumvented or surmounted
2. n. an obstruction that stands in the way (and must be removed or surmounted or circumvented)