Colloquial term for a Ph.D.-trained individual who works as a "post-doctoral fellow" at an academic institution or (sometimes) a company. They are supposedly training to become professors themselves, however there are many more post-docs than there are tenure-track academic positions for them. Post-docing is often a transitional period leading to getting a non-academic job in industry or becoming a writer or any other thing you can do with a Ph.D.

At least in the academic community, post-docs are actually paid less than technicians, and are thus a cheap source of highly-skilled labour.