The term that describes igneous rocks that crystallize inside the Earth is intrusive.

Here's why:

* Intrusive igneous rocks form when magma (molten rock) cools and solidifies beneath the Earth's surface. The slow cooling process allows for the formation of large crystals, giving these rocks a coarse-grained texture. Examples include granite and gabbro.

* Extrusive igneous rocks form when lava (molten rock erupted onto the Earth's surface) cools and solidifies. The rapid cooling process results in small crystals, giving these rocks a fine-grained texture. Examples include basalt and rhyolite.