Chroma refers to the purity of a color. A hue with high chroma has no black, white, or gray added to it. Conversely, adding white, black, or gray reduces its chroma. It’s similar to saturation but not quite the same. Chroma can be thought of as the brightness of a color in comparison to white. Instead, saturation refers to how a hue appears under particular lighting conditions. Think of saturation in terms of weak vs. strong or pale vs. pure hues.

Source: Smashing Magazine