A sign-off in a denomination other than that previously bid by partner or a bid after partner has doubled.
If partner has bid 1NT and you hold a weak unbalanced hand, it is usual to take out into your long suit (via a transfer if you are playing those).
If partner has made a take-out double, you will nearly always take out the double.
If partner has made a penalty double, it will be rather less common to take out the double.