Definition: A double replacement reaction is a chemical reaction where two reactant ionic compounds exchange ions to form two new product compounds with the same ions.
Double replacement reactions take the form:
A+B- + C+D- → A+D- + C+B-
Examples: The reaction
AgNO3 + NaCl → AgCl + NaNO3
is a double replacement reaction. The silver traded its nitrite ion for the sodium's chloride ion.