1. (noun) an arrangement of two or more objects or persons one behind another (tandem)
2. (adverb) one behind the other (tandem, in tandem)
"Ride tandem on a bicycle built for two"
"Riding horses down the path in tandem"
3. (noun) a bicycle with two sets of pedals and two seats (bicycle-built-for-two, tandem bicycle, tandem)