Obstacle, n.
Etymology: [F., fr. L. obstaculum, fr. obstare to withstand, oppose; ob (see Ob-) + stare to stand. See Stand. and cf. Oust, v.]
That which stands in the way, or opposes; anything that hinders progress; a hindrance; an obstruction, physical or moral. If all obstacles were cut away. And that my path were even to the crown. Shak.
Syn. -- Impediment; obstuction; hindrance; difficulty. See Impediment, and Obstruction.
Shades of meaning
Fernald has no entry of its own for obstacle; he lists it under barrier, impediment.
Where he writes about it:
An impediment is primarily something that checks the foot or in any way makes advance slow or difficult; an obstacle is something that stands across the way, an obstruction something that is built or placed across the way. — from impediment
An obstruction is always an obstacle, but an obstacle may not always be properly termed an obstruction; boxes and bales placed on the sidewalk are obstructions to travel; an ice-floe is an obstacle to navigation, and may become an obstruction if it closes an inlet or channel. — from impediment
An encumbrance is always what one carries with him; an obstacle or an obstruction is always without. — from impediment
To a marching soldier the steepness of a mountain path is a difficulty, loose stones are impediments, a fence is an obstruction, a cliff or a boulder across the way is an obstacle; a knapsack is an encumbrance. — from impediment