Transactional Analysis refers a theory of personality and communication, and a psychotherapeutic method.
TA suggests that the personality contains three ego states: 'Parent' (containing instructions and moral codes), 'Adult' (which evaluates and acts on information) and 'Child' (the home of playfulness and creativity). Everyone has and may act from all three, though individuals tend to have preferred modes.
This theory is especially concerned with relationships and transactions and commercial researchers have usefully applied this thinking in a number of ways over the past decades. It can, for example, help understand interactions between members of a group discussion, or help think through people's relationships with brands and/or advertising.