pleural effusion is the collection of fluid other than blood, lymph or pus between the two layers of pleura outside the lung (NB. a collection of blood is a haemothorax, a collection of lymph is a chylothorax while collection of pus is an empyema)
it may be mobile or loculated
it may be caused by transudate or an inflammatory exudate (fibrinous, serous or serofibrinous effusion which may become haemorrhagic or purulent)
if large it may impair respiratory function and require drainage to relieve the pressure upon the lungs