A single-displacement reaction, also called single-displacement reaction, is a type of oxidation-reduction chemical reaction when an element or ion moves out of one compound and into another. (One element is replaced by another in a compound.) This is usually written as... 
'A + BC → AC + B' 
Definition: A double displacement reaction is a type of reaction where part of of one reactant is replaced by part of another reactant. 

Double displacement reactions take the form: 

AB + CD → AD + CB 
The reaction 

AgNO3 + NaCl → AgCl + NaNO3 

is a double displacement reaction. The silver traded its nitrite ion for the sodium's chloride ion.