A large plastic bowl or Tupperware container left on the counter next to the kitchen sink during a house party. The idea being that instead of throwing the last bit of an unwanted drink down the sink, you pour it into the fishbowl to be drank by multiple people with straws later on. Often more than one fishbowl will be created throughout the night, so face-off for downing the whole thing is a common occurrence; often with disastrous consequences.
Alternatively a very strong drink ordered in a bar consisting of all the spirits the bar offers and lemonade, served in a plastic fishbowl and placed into the center of the table to be shared between the group.
John: I don't know what was in that fishbowl last night, but I started blowing chunks after the first few gulps
Alex: We caught Jess spoiling the fishbowl with orange juice last night, so we made her drink a whole pint from it