Singularity
A singularity is an arm pose where axes align and the robot loses a degree of freedom, causing erratic high-speed motion. Paths are planned to avoid it.
A singularity is a configuration in which two or more of a robot's axes line up, so the arm temporarily loses the ability to move the tool in a certain direction. Near a singularity, small Cartesian moves demand huge, fast joint motions.
Wrist singularities (axes 4 and 6 aligned) are the most common. They cause sudden high-speed flips, faults, or stops during programmed paths, which is why integrators plan trajectories to route around singular regions.
Understanding singularities matters when programming any used or new robot — a path that crosses a singularity will misbehave regardless of the robot's condition. It is a kinematic property, not a fault.