Which biconditional is NOT a good definition?

Two lines are parallel if and only if they do not intersect.
A whole number is even if and only if it is divisible by 2.
Two line segments are congruent if and only if they have same length.
Three points are collinear if and only if they lie on the same line.

Question

Which biconditional is NOT a good definition?
 
  
 Two lines are parallel if and only if they do not intersect. 
 A whole number is even if and only if it is divisible by 2. 
 Two line segments are congruent if and only if they have same length. 
 Three points are collinear if and only if they lie on the same line.

directrix · Answer

Parallel lines do not intersect but they also lie in the same plane.

Because of that, the following is an incorrect definition of parallel lines because skew lines are two lines that do not intersect but lie in different planes. The given definition of parallel lines could just as easily be a definition of skew lines. 

 Two lines are parallel if and only if they do not intersect. 
is an incorrect definition for parallel lines 

@stephaniebenitez