Priority seat
What it really means
Seats marked for elderly, disabled or pregnant passengers, usually near the doors. You may sit there when they are free, but you are expected to offer the seat if someone needs it more.
Context, tips & common mistakes
Priority seats — marked with signs near the doors on buses, trains and the Tube — are for disabled passengers, pregnant women, older people and anyone less able to stand. Sitting in one when it is not needed is completely fine; the obligation is releasing it the moment someone who needs it appears, ideally without being asked: stand, gesture, 'here you go'.
Two systems help you respond rather than guess: London's 'Baby on board' badges (free from TfL) remove the mortal danger of guessing at pregnancy, and the sunflower lanyard signals hidden disabilities — chronic pain, fatigue conditions — that explain why an apparently healthy person needs the seat. Which is also the rule in reverse: never police a stranger's use of a priority seat, because need is frequently invisible. If you need one yourself, asking directly ('sorry, could I sit down?') almost always works instantly.