Wiktionary
n. An individual who does not drink alcohol at a social engagement for the purpose of drive his or her companions home.
WordNet
n. the member of a party who is designated to refrain from alcohol and so is sober when it is time to drive home
Wikipedia
The terms "designated driver" and "designated driving" refers to the selection of a person who remains sober as the responsible driver of a vehicle whilst others have been allowed to drink alcoholic beverages.
Thus as a practical and ethical matter a designated driver is a person who abstains from alcohol on a social occasion in order to drive his or her companions home safely as an alternative to driving under the influence. In order to encourage these arrangements, some bar, restaurant and nightclub proprietors will offer free or reduced-price soft drinks to designated drivers.
Usage examples of "designated driver".
Now he was driving this Hawker-Aichi roadster up the Five, very definitely the designated driver, no idea what was waiting for him up there, and halfway wondering if he weren't about to transport a stolen vehicle across a state line.
Definite advantage to the whole chauffeur/bodyguard deal: built-in designated driver.
It had been Damon's shout - two lagers and a Coke, this last for the designated driver.