Obstruction, n.
Etymology: [L.obstructio.]
1. The act of obstructing, or state of being obstructed.
2. That which obstructs or impedes; an obstacle; an impediment; a hindrance. A popular assembly free from obstruction. Swift.
3. The condition of having the natural powers obstructed in their usual course; the arrest of the vital functions; death. [Poetic] To die, and go we know not where, To lie in cold obstruction, and to rot. Shak.
Syn. -- Obstacle; bar; barrier; impediment; clog; check; hindrance. -- Obstruction, Obstacle. The difference between these words is that indicated by their etymology; an obstacle is something standing in the way; an obstruction is something put in the way. Obstacle implies more fixedness and is the stronger word. We remove obstructions; we surmount obstacles. Disparity in age seems a greater obstacle to an intimate friendship than inequality of fortune. Collier. The king expected to meet with all the obstructions and difficulties his enraged enemies could lay in his way. Clarendon.
Shades of meaning
Fernald has no entry of its own for obstruction; 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