The concept of validation comes from Marsha Linehan, Ph.D, a clinical psychologist and creator of dialectical behavior therapy (DBT).
In her 1993 book Cognitive Behavioral Treatment of Borderline Personality Disorder, Linehan notes the essence of validation:
The therapist communicates to the client that her responses make sense and are understandable within her current life context or situation. The therapist actively accepts the client and communicates this acceptance to the client. The therapist takes the client’s responses seriously and does not discount or trivialize them.
Validation is also a powerful parenting tool.