The area in a range of motion where the resistance is furthest away from the center of gravity.
The biomechanically weakest point where you get "stuck" during your rep (usually halfway through). In a bicep curl, for example, the sticking point is likely going to be when the forearm is in a 90-degree angle from the upper arm, as the laws of physics makes it the biomechanically heaviest point.
Most difficult part of a movement.
The point in time when a muscle will resist hypertrophy, no matter how hard you work it. Sticking points are normal. Hypertrophy usually resumes after a short period of dormancy, or if you change your routine.
The point in a movement or exercise through which movement is most difficult. This is especially pertinent in free motion exercises like the squat where posture and body position are so demanding - it is often very tempting to break form in order to pass through the sticking point, and that is where the body becomes most susceptible to precarious positions and potential injury. Another example: The point in the range of motion where the person performing is at a biomechanical DIS-advantage. Example: The 90-degree position in an isolated arm curl.