Laxity. The grading is based upon the degree of laxity noted on physical exam (grade 1-3). It is often compared to the other knee to get a baseline for what is normal for you.
There are 3 grades. A grade 1 tear is a sprain where the mcl is stretched but is not torn. It is swollen and painful but the knee is stable. A grade 2 tear is a partial tear and there may be some slight opening of the joint when stressed. A grade 3 tear is a complete disruption of the mcl and the knee opens medially when forced into a valgus or "knock-knee" position.