Saddle pulmonary embolus.
A thrombus is a blood clot that is localized to a site; an embolus can be thought of as a break off product that is mobilized away from its site of origin by circulation. There are many kinds of emboli. They can be fat, air, tumour metastasis, bone marrow and more commonly blood.
A blood clot can get quite large and come in various shapes and sizes. This one is an example of a large blood clot that is saddling the two pulmonary arteries but not completely obstructing blood flow to either side.