A long long time ago, in the early 1920’s, dimensional ideas about mood disorders started to formulate. Schizophrenia and Bipolar both got put into spectrums (not the same one) and people thought about them in terms of their severity and intensity as opposed to a simple ‘yes/no’ diagnosis. These models were massaged and changed over the decades as our understanding grew and this continued until the DSM was created. The DSM (Diagnostic and Statistical Manual of Mental Disorders) was first published in 1952, and has since had 4 (and two sub) revisions. DSM-5 was published in 2013.
And none of them have defined bipolar or schizophrenia as a spectrum disease, like autism for example.
The DSM is used in a ton of countries (over 60), even though it is written by the APA. There is a counterpart out there called the International Statistical Classification of Diseases and Related Health Problems ICD-10, which is put out by the World Health Organization. For whatever it matters, on an international scale, professionals prefer to use ICD-10 for clinical diagnosis, while the DSM-IV is considered better for research. Here in America, DSM is the standard and is used for everything, and is necessary for insurance coverage.