A solution to an equation that SEEMS to be right, but when we check it (by substituting it into the original equation) we find it is NOT right.
Example: you work on an equation and come up with two roots (where it equals zero) "a" and "b".
Putting "a" into the original equation makes it zero.
But putting "b" into the original equation does NOT make it zero.
So "b" is an extraneous root.
This often happens when we square both sides during our solution.