A half-gallon (64 ounce) jug sold by small breweries and brewpubs for take-out purposes. Typically adorned with the label of the brewery/establishment it came from. Most breweries/brewpubs will only refill their own growlers. Not to be confused with it's little cousin the 32-ounce howler.
"Where's your growler?" (As used in Russ Meyer film, 1969 by Charles Napier, actor of sorts, enquiring for the lavvy at a petrol station - or gas if you prefer).