Axis
An axis is a single controlled joint of a robot, adding one degree of freedom. Standard arms have six, SCARA four, plus optional external rails and positioners.
An axis is one independently driven and controlled joint of a robot. Each axis provides one degree of freedom — a rotation or, less commonly, a linear movement. The number of axes determines how flexibly the robot can position and orient its tool.
A standard industrial arm has six axes; SCARA robots have four; some robots add a seventh for reach-around flexibility. External axes (turntables, linear rails, positioners) extend the system beyond the arm and are coordinated by the same controller.
When evaluating a used robot, count the axes and confirm each moves smoothly across its full range without backlash. Worn axis bearings or gearboxes are a leading cause of lost repeatability.