You're close! While "isthmus" and "peninsula" are terms that relate to landforms, they don't necessarily imply "flat land". Here's a breakdown:

* Isthmus: A narrow strip of land connecting two larger landmasses. It's like a bridge between continents or large landforms. The Isthmus of Tehuantepec in Mexico is a great example, connecting North America to Central America.

* Peninsula: A piece of land surrounded by water on three sides, with one side connected to a larger landmass. Think of it as a finger of land extending into the sea. Mexico itself is a peninsula! Other examples are Baja California and the Yucatan Peninsula.

Neither an isthmus nor a peninsula inherently needs to be flat. They can be mountainous, hilly, or flat.

Here's how to think about it:

* Isthmus: A narrow "bridge" of land between two larger masses

* Peninsula: A "finger" of land extending into the sea

Mexico has both, but the terrain within those features can vary greatly.