The defining data of a reflection is a line m, called the mirror of the reflection
If the point X lies on the mirror line m, then its image point X' is equal to X.
If the point X does not lie on the line m, then its image point X' is determined by the property that the line XX' is perpendicular to the line m, and the distance from X' to m equals the distance from X to m. In other words, if P is the intersection point of the line XX' and the line m, then d(X',P) = d(X,P). A concise way to say this is that the mirror line m is the perpendicular bisector of the segment XX'.