What Is a Triangle Base?

The base can be any side of a triangle

To find the area of a triangle, we multiply $\text{base} \times$$\text{height}$ and then divide by $2$. Here, $\text{base}$ refers to any side of the triangle, while $\text{height}$ refers to the length of the altitude to that side.

So to summarize again: any side of any triangle can be the base. All the sides of the triangles in the images below can be considered bases.

However, we also sometimes refer to the single unique side in an isosceles triangle as the base. Math is weird.