impediment
Something that seriously slows down or blocks progress.
An impediment is something that blocks your progress or makes it harder to do something you're trying to accomplish. It slows you down or stops you from moving forward.
If you're trying to finish your homework but the power goes out, that's an impediment. If a soccer team wants to win the championship but their star player is injured, the injury is an impediment to their goal. When a student has a speech impediment, something makes it physically harder for them to pronounce certain sounds or speak smoothly.
Notice that an impediment doesn't make something impossible, just more difficult. A heavy backpack might be an impediment to running fast, but you could still run if you really needed to. Thick fog is an impediment to driving safely, so drivers slow down and turn on their headlights.
The word suggests something substantial and real, not just a minor annoyance. You wouldn't call a small distraction an impediment, but you would use the word for a serious obstacle. When scientists discover a major impediment to their research, like equipment that keeps breaking down, they have to find creative ways to work around it or solve the problem.