A sideways drift that may be experienced by helicopters with a tail rotor, when they are hovering.
As you may or may not be aware, a helicopter supports itself in the air by spinning a set of rotor blades that have wing cross sections. The rotors spinning creates lift which enables the helicopter to fly. In accordance with Newton's third law of motion, in spinning the rotors the helicopter creates an equal, opposite force on itself. This means that while the main rotor is spinning one direction, the helicopter fuselage 'wants' to spin in the opposite direction.
Of course, ignoring this immutable principle would result in the antithesis of controlled flight, so a helicopter must have a way to counteract it. There are several methods of doing so but the majority of helicopter designs employ a tail rotor. Another, smaller set of rotor blades are mounted sideways on the end of the helo's tail boom. When they rotate they create thrust which opposes the torque being generated by the main rotor as it spins. This writeup also applies to NOTAR (NO TAil Rotor) systems as they have the same effect as a tail rotor.
If the main rotor turns clockwise, it makes the helicopter fuselage 'want' to turn anticlockwise. The tail rotor creates equal thrust in the opposite direction, thus the helicopter does not spin wildly. It also provides yaw control, but that's another node. The tail rotor actually spins about six times faster than the main rotor, to provide sufficient thrust without having to be very large.
Onto the meat, such as it is. While the tail rotor is an effective, albeit mechanically complex method of torque compensation, it has a side effect. The tail rotor is there to counteract the torque force generated by the main rotor; however it also generates a lateral force, or 'sideways lift' which affects the fuselage. Thus, when a helicopter is hovering (ignoring wind for simplification) the tail rotor will push the helicopter in one direction.
On most modern helicopters the main rotor turns counter clockwise, meaning it exerts a clockwise turning force on the helicopter fuselage. The tail rotor, opposing this force, exerts an equal but opposite (counter clockwise) turning force on the fuselage. It also exerts some lateral force on the fuselage in the same direction. When hovering with the cyclic control (the pilot's control stick) in a neutral position, the helicopter will tend to drift slowly to the pilot's left. This is translation tendency. If the main rotor turns clockwise, the helicopter will tend to drift to the pilot's right.
One might think that as a known quantity, helicopters would have automatic cyclic compensation for this force. In fact it is such a minor effect (at most it will make the helicopter drift a couple of inches per second) the pilot will normally manually compensate with some opposite cyclic; this means that when a helicopter is hovering it will be slightly tilted to one side, one skid lower the the other. If you ask a helo pilot how he's doing and he replies "left skid low," everything's hunky dory.
Some large helicopters do have automatic compensation for translating tendency, but these apply a fixed amount of opposite cyclic so do not always compensate the correct amount. The amount of compensation required depends on the weather conditions and the helicopter load, so the pilot will always have to do a certain amount of manual compensation anyway.