To master the deadlift, RDL, or kettlebell swing, you have to know how to hip hinge. Though there are hingy squats and squatty hinges, the two are not the same. They have distinct characteristics that allow us to categorize other exercises as forms of each. Though the biomechanics of the squat and hinge are complex, the MAIN differentiating factor is the joint that each one emphasizes.
The joint plays a primary role in the execution of each pattern. So remember this:
A squat pattern will place more emphasis on the knee joint:
A hip hinge pattern will start at the hip and place more emphasis there, hence the naming of the pattern itself:
While there's obviously movement happening at both the hips and knees, the primary joint leading the movement itself is the distinguishing factor when grouping the patterns.