Problem is a noun that refers to a matter or situation regarded as unwelcome or harmful and needing to be dealt with and overcome.
In mathematics, a problem is a question proposed for solution or consideration.
The word "problem" can also be used as an adjective meaning difficult to deal with or control.
Common collocations:
1. No problem - used to express that something is not difficult or troublesome
2. Problem child - a child who is difficult to control or teach
3. Problem-solving - the process of finding solutions to difficult or complex issues