There isn't a specific term for a mix between a rainforest and a plain. This is because there are many ways a rainforest and a plain could mix, resulting in different ecosystems with various characteristics.

Here are some possibilities:

* Ecotones: These are transition zones between different ecosystems. You could have an ecotone between a rainforest and a plain, which would have characteristics of both.

* Savanna: Savannas are grasslands with scattered trees, which could be seen as a transition between a rainforest and a plain, especially in areas where rainfall is decreasing.

* Tropical Dry Forest: These forests experience seasonal rainfall and can be found along the edges of rainforests. They can be considered a mix of rainforest and plain due to their less dense canopy and open areas.

* Montane Forest: These forests occur at higher elevations and may have characteristics of both rainforests and plains, depending on the specific location and altitude.

Ultimately, the specific characteristics of the mix depend on the location, climate, and other environmental factors. It's important to consider the specific details of the ecosystem to understand its unique features.