balk vs obstacle

balk and obstacle both are nouns.

Word	Noun	Adjective	Verb	Adverb
balk	Yes	No	Yes	No
obstacle	Yes	No	No	No

As nouns, obstacle is a hyponym of balk; that is, obstacle is a word with a more specific, narrower meaning than balk:

balk: something immaterial that interferes with or delays action or progress
obstacle: something immaterial that stands in the way and must be circumvented or surmounted

Other hyponyms of balk include albatross, millstone, bind, diriment impediment, drag, obstruction, straitjacket.

balk (noun)	obstacle (noun)
an illegal pitching motion while runners are on base	an obstruction that stands in the way (and must be removed or surmounted or circumvented)
one of several parallel sloping beams that support a roof	something immaterial that stands in the way and must be circumvented or surmounted
something immaterial that interferes with or delays action or progress
the area on a billiard table behind the balkline

balk (verb)	obstacle (verb)
refuse to comply