A Single Subject Teaching Credential is a Californian teaching credential that authorizes the holder to teach a specific subject in a classroom. Examples of holders would be an English Teacher, Science Teacher, Math Teacher, etc. These are most commonly for secondary educators in middle and high school where the classes switch each period, meaning the teacher is ONLY teaching their subject the whole day while the students hop around subject to subject.
To acquire a single subject credential, the holder must not only complete a teacher preparation program from an accredited university, but must also demonstrate subject matter competency in the form of a degree acceptable to the university in question, or else take the CSET-- the California Subject Examinations for Teachers. An example: I received my English degree at the same university where I completed my teacher preparation program, so they accepted that I had subject matter competency. However my friend completed her science degree at a different university out of state, and so she needed to prove her subject matter competency with the CSET scores.