Replacement hub and mid-drive motors and conversion kits.
E-bike motors are tough. A well-kept hub motor will outlast the battery several times over, and most motor problems that look like a dead motor turn out to be a connector, a hall sensor wire, or the controller. But motors do fail: water gets past the seals, bearings wear, internal gears strip in geared hubs, and phase wires chafe where they exit the axle. Motors are also the heart of a conversion kit if you are electrifying a regular bike, and that is where most of the buying happens.
The first split is hub versus mid-drive. Hub motors sit in the front or rear wheel, are simple and affordable, and come as a complete laced wheel or as a bare motor to lace yourself. Within hubs, geared units are lighter, freewheel when unpowered, and punch above their weight at low speed; direct-drive units are heavier and quieter, can regenerate a little on braking, and drag slightly when coasting. Mid-drives sit at the bottom bracket, drive through the bike's gears, and climb better, but they wear chains faster and are a bigger installation job.
Whatever you buy must match the controller it will run on: voltage, current, and sensor wiring (hall-sensored or sensorless, and the connector on the motor cable). It also must physically fit the frame: dropout width, axle size, brake mount, and on a rear hub whether the bike uses a threaded freewheel or a cassette. Replacement motors are best bought as the exact same model, or as a kit with a matched controller. Mismatched electrical parts can damage a controller or battery; if you are not sure what you have, an E-Bike Assessment is the safe next step.