Relationship and differences between psychosomatic, somatoform and idiopathic disorders I'm really confused because it seems to me these terms overlap to some extent.

Psychosomatic disorder


  psychosomatic adj.
  1. of or pertaining to a physical disorder that is caused or notably influenced by emotional factors. American Heritage Dictionary
  
  psychosomatic
  1. (Psychology) of or relating to disorders, such as stomach ulcers, thought to be caused or aggravated by psychological factors such as
  stress Collins English Dictionary


somatoform disorder (called somatic symptom disorder since DSM-V)


  denoting physical symptoms that can not be attributed to organic
  disease and appear to be of psychic origin.  Miller-Keane
  Encyclopedia and Dictionary of Medicine, Nursing, and Allied
  Health
  
  denoting physical symptoms that cannot be attributed to organic
  disease and appear to be psychogenic. Dorland's Medical
  Dictionary for Health Consumers


In both cases these terms seem to be used for physical symptoms which rather than being attributed to physical conditions, are based on mental factors, ie., psychogenic.

Also, three of these definitions are framed in terms of "thought to be caused by psychological factors", "appear to be of psychic origin" and "appear to be psychogenic." Given that these three terms imply cause is not known with certainty they are idiopathic, right?


  idiopathic adj. Of, relating to, or designating a disease
  having no known cause. American Heritage Dictionary


From looking at these two meanings of psychosomatic disorder and somatoform disorder, am I mistaken in perceiving them the same way, or at least confusingly almost the same thing? One says "relating to disorders ... thought to be caused or aggravated by psychological factors" and the other says "denoting physical symptoms ... (that) appear to be of psychic origin."

Could someone clarify what the differences are in light of the definitions, which seem quite the same to me?
This is purely all about terminology.

Somatic means of the body; bodily; physical. whereas the psycho part of psychosomatic refers to the fact that the somatic problem (symptom or disorder) is created through psychological means (with no medical evidence of a physical problem e.g. broken bone and/or torn ligaments).

Symptom refers to an observable behaviour or state, whilst disorder refers to a cluster of symptoms.

Somatoform refers to a physical symptom that cannot be attributed to organic disease and appears to be psychogenic (Dorland's Medical Dictionary for Health Consumers, 2007). 

Somatoform disorder refers to a set of psychosomatic symptoms (somatoforms) (Smith &amp; Józefowicz, 2012).

Idiopathic, as you pointed out is


  Of, relating to, or designating a disease having no known cause.


Further clarification

Psychosomatic is a word describing a somatoform.  A psychological problem is a somatised (Al Busaidi, 2010), and therefore is producing the experience of somatic symptoms, which are in fact psychogenic.

If a medical problem is considered idiopathic, the issue of whether it is somatic or psychosomatic is not determined. It cannot be both psychosomatic and idiopathic at the same time.

References

Al Busaidi, Z. Q. (2010). The concept of somatisation: a cross-cultural perspective. Sultan Qaboos University Medical Journal, 10(2), 180. Pubmed: PMC3074701

Dorland's Medical Dictionary for Health Consumers. (2007). Retrieved from: https://medical-dictionary.thefreedictionary.com/somatoform

Smith, J. K., &amp; Józefowicz, R. F. (2012). Diagnosis and treatment of somatoform disorders. Neurology: Clinical Practice, 2(2), 94-102. doi: 10.1212/CPJ.0b013e31825a6183