In the basement of a condominium, there is a duct that leads from the outside into the basement. Looking from the basement, this duct is connected to nothing. What is the purpose of this duct?

Could be a fresh air make up or could be for combustion air. It is there for a reason so do not mess with it.

If you are concerned then bring in a pro who can go through all the possibilities such as hvac1000 mentioned. Make sure you cover all bases as this could even be some kind of Radon abatement solution.