The opening in the Earth's crust through which lava flows is called a volcano.

More specifically:

* Vent: The actual opening where the lava erupts.

* Crater: The bowl-shaped depression at the summit of a volcano.

* Caldera: A large, circular depression formed when a volcano collapses into its own magma chamber.

So, while "volcano" is the most common and encompassing term, "vent" is the exact spot where the lava emerges.