Gravamen is a noun that refers to the most significant part of a complaint. 

The Latin word gravis (GRAH vees) means ‘heavy.’ shifting into English in the early 17th century, our word of the day is used to describe the weightiest or most important part of a person’s complaint. Here’s an example:

I have to admit I didn’t care for the lame music playing in Dr. Simon’s office, but I wouldn’t say it was the gravamen of my complaint regarding my experience there. My biggest problem with the visit is that he spent two hours drilling into my gums.