What is the best definition of the phrase a bone of contention

A 'bone of contention', is the major cause of dispute or conflict between two or more people.

Explanation:

The phrase, 'bone of contention', is an idiomatic expression that designates a factor as the major cause of a conflict. An idiomatic expression is one whose meaning may not be easily deduced from the literal words used. The literal meaning of this phrase is a bit transparent from the words used. For example, contention gives the idea that there is a conflict.

An example of a statement, where the phrase, 'bone of contention' is used is this: Kelvins' property was the bone of contention among his children. In this instance, there is a conflict among Kelvins' children, and his property was the cause of the conflict.