"Psychology"

Morphology
psych < Greek psyche = soul, life, person
o = connecting vowel
log = say
y = process
(logy = discussion)

"psychiatry"

iatr = healer
y = process

Psychology, from its etymology, should mean "a discussion of the soul." However, it has come to refer to the study of behavior (quite a different matter). I believe the reason for the change in meaning is that first of all it was thought that the real person was his soul and that his soul directed his body. Thus an explanation for behavior would refer to the soul.

However, with the modern denial of the existence of any soul (in favor of materialism), yet persons wanting to be gurus and receive money as experts, kept the name "psychologist," and the name for their field, psychology. Thus, IMO, the use of these terms is based on dishonesty.

Psychiatry, etymologically, should mean to the science of curing souls. But the same problem as above occurs; persons wanting to be gurus in this field are likely not to believe in any soul.

In practicality, psychiatry is a term applied to the practice of medical doctors, who may write prescriptions for "mental illnesses," even if they don't believe in a nonmaterial mind; but just a brain.

Psychology, then, is used for the field in which therapies other than prescriptions are used to treat "mental illnesses," though psychology is broader in scope that just "mental illnesses," which come under "abnormal psychology." Non-psychiatric psychologist use talk therapy (counseling) and sometimes behavioral reinforcement schedules to modify behavior (rewards vs. punishments).

To have a mental illness/disorder (generally speaking) as defined in the DSM-IV, Diagnostic and Statistical Manual of Mental Disorders (the psych insurance Bible), 1 of 2 things must exist:

1) The person is bothered by the behavior or
2) The person's functioning in society is impeded by the behavior.

Otherwise, you can be as nutty as the mad-hatter and not be diagnosed as "mentally ill," since your behavior neither bothers you yourself, nor impedes your functioning.

Also, you can get a pass on mental illness if the behavior(s) is / are a part of your religion or culture.

"Biblical psychology" is the study of and employment of what the Bible says about the soul (and similar concepts). This includes the study of certain Greek and Hebrew words in the Bible in their contexts, words that translate into English as soul, spirit, mind, heart, flesh (= human nature), and the like. For example, Ezekiel writes about a time when the Lord will remove the heart of stone and give a man a heart of flesh instead. Romans 12 speaks of being transformed by the renewal of one's mind. Galatians 5 speaks of Walking by the Spirit and not fulfilling the lust of the flesh.