The rock that existed before it changed into a metamorphic rock is called the protolith.

Think of it like this:

* Protolith is the "original" rock.

* Metamorphic rock is the rock that is formed after the protolith undergoes changes due to heat, pressure, and/or chemical reactions.

For example, if a sandstone undergoes metamorphosis, it can become a quartzite. The sandstone is the protolith, and the quartzite is the metamorphic rock.