| To piggyback is to have Alpha seated on Beta's backside, with Alpha's thighs straddling Beta's hips. Normally, Alpha will hold Beta around the chest (or less desirably, the neck), and Beta will further support Alpha's weight by holding the thighs or knees. If the pair should walk, run, or dance about, this would be referred to as a piggyback ride.
Although both persons can be said to "piggyback", it can be clarified who serves which role in the physical relationship by having Alpha "be given a piggyback by" Beta, and conversely by having Beta "give a piggyback to" Alpha. (Interestingly enough, the role of Alpha is both dominant and submissive.)
Contrast with shoulder ride.
Piggybacking is also used to describe means of transportation which involve a carried mass atop a locomotive mass, as in trains, trucks, and rocket ships.
On a more abstract level, piggybacking is used to descibe an effect which is realised because of another effect, as in chemical processes, governmental legislations, medical trauma, or computer data transfer. | 
