Frequency: The word is one of the 3000 most common words in English.
1.
A ~ problem or situation is very difficult to deal with and shows a lot about the character of the person who is dealing with it.
The most ~ time is undoubtedly in the early months of your return to work...
ADJ
2.
Testing is the activity of ~ something or someone in order to find out information.
The National Collegiate Athletic Association introduced drug ~ in the mid-1980s.
N-UNCOUNT