A short, everyday form of 'rehabilitation'. Its most common meaning is treatment for someone with a drug or alcohol problem — and it usually names the PLACE as much as the process. 'He's in rehab' means he is staying at a treatment centre; 'she went to rehab' means she checked into one. Because of that, it is normally used without 'the': in rehab, not in the rehab.

It is informal but not rude, and it is completely standard in news, in conversation, and in the way celebrities' lives get reported. The full word 'rehabilitation' is what appears on the clinic's sign; 'rehab' is what people actually say.

Two other uses are worth knowing. In medicine and sport it means the exercises and therapy you do to recover the use of your body after an injury or operation — an injured player 'is doing rehab on his knee'. And in building, it means repairing and modernising an old property; as a verb, 'they rehabbed the whole house' means they renovated it. That verb use is much more common in American English, especially in property talk.

A small note on feeling: rehab carries less shame than it once did, but it still points at a real difficulty in someone's life. Around someone actually going through it, most people speak more gently — 'getting treatment', 'getting help'.