When you look at a rainbow, you will notice that the colors blend seamlessly into one another. You don’t see a line dividing the colors.
The color wheel with secondary colors is divided into 12 segments. But you can divide a color wheel down even further… reducing the clear definition between colors.
When you mix primary colors in even parts, you get secondary colors. And if you mix a primary color with a secondary color, you get a tertiary color.
Tertiary colors are hues that fall directly halfway between a primary color and a secondary color. If you look at the color wheel on this page, the tertiary colors are the pie pieces that extend outside of the circle.
As you can imagine, there is infinite number of colors that fall between a primary color and a secondary color. As a general term, these are called intermediate hues.