servomotor
A servomotor is a motor that knows where it is and corrects itself to get where you want it. Plain motors are told only "spin this fast" and hope for the best; a servomotor is told "go to this exact angle" or "hold this exact speed," and it keeps checking and adjusting until it is right. It is a motor packaged with a built-in sensor and a controller that work in a constant loop — command, measure, correct — to hit and hold a target.
That loop is the heart of it, and engineers call it closed-loop control. A position sensor (an encoder) reports where the shaft actually is; the controller compares that against where it was told to be; and the difference, the error, is used to drive the motor harder or ease it off until the error shrinks to nothing. If something shoves the load off target, the servomotor pushes right back to restore the commanded position. This self-correcting habit is exactly what an ordinary motor lacks and what makes a servo trustworthy for precise, repeatable motion.
Servomotors come in two everyday flavors. The little hobby servo — the kind in a model plane's wings or a beginner robot arm — is a small geared motor that swings to a commanded angle and holds it. The industrial servomotor is a powerful, precise machine, usually a brushless motor with a high-resolution encoder, that drives factory robots, CNC machines, and robot joints to exact positions, speeds, or forces. Either way, the defining idea is the same: feedback turns a dumb spinning motor into one that hits its mark and refuses to drift off it.
A robot arm must place a chip on a circuit board to within a hair's width. A servomotor at each joint reads its encoder, sees it is a fraction of a degree off, and nudges itself until the tip lands exactly on the target — then resists any bump that tries to push it away.
Command, measure, correct — the loop that lets a robot arm hit its mark every time.
"Servomotor" describes a role, not a single design: it is any motor wrapped in a feedback loop. The control math that does the correcting — most often PID — belongs to classical control, covered there.