A setback is an event or situation that makes progress slower or stops it completely. Imagine running towards your goal, but suddenly hitting a roadblock.
When we talk about setbacks, we usually mean situations that hinder our plans or goals. For example, if you're trying to learn a new language and you have a bad day of forgetting vocabulary, that's a setback. Or, if you're training for a marathon and you get injured, that's definitely a setback. Setbacks can be small, like missing one day of work due to illness, or big, like losing your job because of company restructuring.
An obstacle, delay, disadvantage, blow (an adverse event which retards or prevents progress towards a desired outcome)
Example: After some initial setbacks, the expedition went safely on its way.
