Technically, a power source and a conductor. The power source may be a battery, wall plug, or other item that can produce electricity. The conductor can be any material that will conduct electricity; wire, buss bar, yourself, any metal or even water (preferably salt water). Connect the conductor from one side of the source to the other and voila! a complete circuit. It won't last long without some form of resistance in the conductor, but it is a complete circuit.
If you don't believe water will conduct electricity, fill a house receptacle with water and call the fire department and rescue squad. I have seen 3' flames from a floor receptacle flooded from a broken sprinkler pipe and people die all the time from appliances or hot wires falling into water with them.