The English word "challenge" can be translated to Spanish as desafío (desafiar) or reto (retar). Is there any difference between these words, or are they exact synonyms? If there is a difference, when is each used, or what is the difference in meaning?
I disagree with Rafael. Although both are in many countries considered as synonyms it really depends on context and there are some inherent subtilities that distinguish them from each other.
Here are the RAE definitions for both:
First the subtilities and then the old meanings. For example you can see on meaning 5 of reto that reto is something that constitutes an objective that is difficult to achieve, that it is a desafío to someone, so the meaning of desafío that is taken here is the one of rivalry and competition. Desafío is something you want to achieve, and reto is something that can be compared with a goal or an obstacle. A difficulty. To help clear this difference: all desafíos are retos but not all retos are desafíos.
Edit For example:
Here reto is in the sense of an obstacle that you have no choice but to surmount. Something tough and difficult unrelated to a goal. The goal is to continue through this way, but the reto is destroying the wall.
Here reto is something difficult to do. And the other person asks if it is a challenge that he is posing to him. So if the other person accepts it is a desafío because it becomes a goal.
Another difference that I just noticed and is new to me is regañina which means to ground someone. To tell a child that he was bad for example. I am unaware of in which region is this word used, but I can guarantee that it is not used in Ecuador or Colombia.
So second, there are some old meanings in which it is specified that in royalty desafío was used as a missive of the king of Aragón to challenge a noble, richman or knight. And reto was an accusation of a noble against another noble made in front of the king, obliging him to face him on the field. So desafío is a challenge of the king against a noble, richman or knight, and reto is noble vs noble in court. (As I understand it)
They are pretty much synonyms, so there is no specific situations where you should use one instead of the other.
"Desafío" is equivalent to the English word "defy," which is to say that it is a challenge thrown down by another person. "Reto" has more of the connotation of "obstacle," something you want to overcome, but maybe set by "nature" rather than another person.