Cum-dividend trading is dealing in a share during the period in which it carries the right to an announced dividend. "Cum" is Latin for "with". A share trades cum-dividend from the announcement until the close of business on the day before the ex-dividend date; a buyer in that window will be the registered holder on the record date, provided the trade settles on the normal cycle, and so receives the payment. From the ex-date onwards the share is dealt without the dividend, as described under ex-dividend trading.
The cum-dividend price includes the value of the coming payment, so the share price usually falls by about the dividend amount when it goes ex. If a cum-dividend trade settles late, the seller may receive a dividend that belongs to the buyer, and the amount is passed on through a market claim.