The phrase "elbow joint" refers to the point where your upper arm bone and two bones in your forearm come together, allowing your arm to bend and straighten. Full definition
For a safe press, keep the elbows tucked at 45 degrees, but avoid having a too narrow grip — try to keep the wrists and elbow joints directly underneath the bar. (fitnessandpower.com)
The four types of elbow joint problems include ununited anconeal process (UAP), fragmented coronoid process (FCP), osteochrondrosis dissecans (OCD) and «elbow incongruency.» (healthypets.mercola.com)