root [countable]
1plant the part of a plant or tree that grows under the ground and gets water from the soil:
tree roots
These plants produce a number of thin roots.
2cause of a problem the main cause of a problem
be/lie at the root of something (=be the cause of something)
Allergies are at the root of a lot of health problems.
The love of money is the root of all evil.
A competent mechanic should be able to get to the root of the problem (=find out the cause of a problem).
the root causes of crime