A psychologist was someone that diagnosed and treated mental disorders in sentient lifeforms.
Dr Susan King was a psychologist and a field agent for the Ministry of Intelligence in the 21st century. (PROSE: The Baron Wastes)
Christine Lafayette, Paula Engado, Shmuel Zehavi and Filo Julee were psychologists on Project Eden. (PROSE: Lucifer Rising)
Sigmund Freud was a 20th century Earth psychologist. The Eighth Doctor visited Freud while the Doctor was suffering from amnesia, in an attempt to jog his own memory. (PROSE: The City of the Dead)