Somebody who has recently completed a PhD and has no idea what do with their life after discovering they are completely unemployable. Anyone with the title "Post Doc" is a bright intellectual person without direction who is killing time waiting for the right job opportunity.
She was a postdoc at the university for ten years before she discovered that she had to find a permanent job to pay for the kitty litter to keep her family together.
A postdoc is a Ph.D. holder who has not been able to secure a stable academic position and therefore sticks around in different labs. A postdoc often lives off soft money in a series of 2-/3-year appointments, until he or she lands a tenure-track position, leaves academia for an industry job, finds a room in the parents' basement, or commit suicide in despair.
Jane: "You know, something's eating away that leftover muffins on the table. Must be mice or roaches hiding in this lab."
Tom: "No, that's our postdoc John."