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In a double displacement reaction, also known as a double replacement reaction, the defining characteristic is that the reactants exchange components with one another. This means that ions or molecules in the reactants switch places to form new products, leading to the creation of different compounds.

For example, if we have two compounds, AB and CD, in a double displacement reaction, they may react to form AD and CB. This switching of parts is crucial and highlights the nature of double displacement reactions distinctly.

The other options might not accurately represent double displacement reactions: involving only two reactants is too restrictive, as more complex reactions can certainly participate; forming only one product is not a requirement, since multiple products can potentially be formed; and while gases can be produced in some reactions, it is not a universal result for all double displacement reactions. Therefore, the hallmark of these reactions is indeed the exchange of parts between reactants.