Synthehol (a portmanteau of "synthesized" and "alcohol") is a chemical variant of alcohol. It appears to have the same taste and smell as "real" alcohol to most individuals, but none of the deleterious effects associated with alcohol for most humanoids, such as debilitating intoxication, addiction, and alcohol poisoning. Most humanoids have an enzyme which breaks down the alcohol-like compounds in synthehol. According to Data, synthehol's "intoxicating effects can be easily dismissed." (TNG: "Relics")
Synthehol is a regular ingredient in the drinks served on Starfleet vessels and starbases. Some bartenders like Guinan and Quark, however, keep a supply of genuine alcoholic beverages for customers requesting such. (TNG: "Relics"; DS9, various)
The slight chemical difference between synthehol and real alcohol is an annoyance to lovers of alcoholic beverages such as Robert Picard and Montgomery Scott. Robert Picard believed that synthehol corrupted his brother Jean-Luc Picard's taste for wine, while Jean-Luc believed it gave one a greater appreciation for the real thing. (TNG: "Family") Captain Scott was able to differentiate alcohol from synthehol when he drank a glass of scotch in the USS Enterprise-D's Ten Forward lounge. He found the taste revolting, and he let the young bartender who served him know it. (TNG: "Relics")
In 2369, during his brief tenure as Grand Nagus, Quark was approached by Nava, who wanted to introduce synthehol to the Gamma Quadrant. Quark thought it sounded like a "very lucrative opportunity." (DS9: "The Nagus")
On at least two occasions, Seven of Nine become intoxicated after drinking synthehol, as her Borg implants were not designed to process it. She suffered similar symptoms to those that alcohol has on Humans. (VOY: "Timeless", "Body and Soul")
In 2375, the Kadi ambassador Tomin visited USS Voyager, attended a reception, and drank some syntheholic beverages. It was later realized that Kadi physiology lacks the enzyme which breaks down the alcohol-like substance in synthehol, which caused him to become drunk and attempt to woo Seven of Nine. He later passed out, and The Doctor was able to remove the synthehol with Seven's nanoprobes. (VOY: "Someone to Watch Over Me")
On some replicators, a synthehol button was available for easy manual input.