To understand the concept of forest in discrete mathematics, we must first understand what a tree is.

A connected undirected graph without loops is called an undirected tree.

Then there is the definition of forest:

An undirected graph in which every connected branch is a tree is called a forest. Each connected branch is a part of a tree. There is a question, can a tree be regarded as a forest?

From the definition of forest, it can be clearly judged that a tree can be called a forest.