Alcohol or alcoholic beverages are drinks served during certain social occasions and/or recreational occasions.
Due to the side effects of each drink they are declared off limits to those underage and have been banned altogether during the Prohibition Era (from 1919 to 1933). Though such bans don't prevent individuals from trying to acquire or distribute them.
Edna Strickland arrived sometime during 1876 from 1931 in a faulty DeLorean time machine, and found herself wanting to remain what she deemed to be a relatively peaceful era of Hill Valley. Due to not approving of the Saloon's selling of alcohol, she planned to burn it down and originally succeeded in doing so until Dr. Emmett Brown and Marty McFly arrived from 1931A to stop her.
When Marty arrived in the Saloon during September 3 of the year, he found himself not wanting any of the drinks as they were all alcoholic. Some time afterward, Doc tried using a strong alcohol concoction to fuel the DeLorean time machine in lieu of gasoline, but it resulted in an explosion that destroyed the fuel injection manifold. During the 7th, a heartbroken Emmett Brown asked for some whiskey, but refrained from drinking it until Marty reminded him of the train they needed to catch.
During 1931, Irving "Kid" Tannen was one of many individuals who distributed alcoholic drinks during Prohibition through some of his speakeasies, such as El Kid A vigilante known as the speakeasy arsonist went about burning each establishment down, though they ceased altogether when Kid was arrested on August 25.
During her teen years in 1955, Lorraine Baines developed an addiction to alcoholic beverages (that she swiped from her mom's liquor cabinet) which carried over all the way to 1985, until Marty's arrival in the 1950s prompted her to end that habit. In addition, Biff Tannen and his gang had a tendency to drink unsupervised, as well as having spiked the punch at the Enchantment Under the Sea dance.
On October 25 in the original 1985, Biff got into an accident while drinking and driving (at the same time!) behind the wheel of George McFly's car. After having chewed out George for not mentioning that the car had a blind spot, as well as forcing George to pay up to have his suit dry-cleaned as he had spilt beer over himself in the crash, Biff helped himself to some beer in the McFly house (while complaining it was Lite).
As the Hill Valley of 1986B had stricter policies, alcoholic beverages were forbidden to any citizen. Marty, wanting to gain an audience with the Citizen Emmett Brown, deliberately broke the rule by finding a flask of booze that Lorraine was drinking.