Is pulmonary embolism an obstruction in the main artery to the lung?

Any artery: A pulmonary embolism (pe) is a clot in any of the arteries in the lungs. If the clot is large, it will lodge in one of the larger arteries, and can be fatal. If the clot is smaller, it will travel into the lung until the vessel is small enough to trap it, and may not even cause much in the way of symptoms. Any pe is serious, and the source of the clot needs to be found to prevent recurrences.

No: A clot travelling through the pulmonary artery may lodge anywhere between the pulmonary artery and its distal branches depending on size of the clot, size of the vessel and friability of the clot. A large clot may lodge in the "main" pulmonary artery and produce sudden death, or it may breakup on its journey and produce a shower of multiple small clots in more distal vessels with minimal symptoms.

Pulmonary embolism: Is a clot/thrombus with the main pulmonary artery on one of its branches.

Yes: Pulmonary embolism is an obstruction, usually caused by a blood clot, in the arteries that go from the right side of the heart to the lungs. Does not have to be in the main artery necessarily.