Hue
The color family itself, red, blue, yellow and so on, or in color analysis, the warm-to-cool axis.
Hue is one of the three dimensions borrowed from the Munsell color system, alongside value and chroma, and it is simply the characteristic that separates one color family from another, red from blue, yellow from green. In seasonal color analysis, hue usually refers more specifically to temperature: the more yellow a color carries, the warmer it reads, and the more blue it carries, the cooler it reads. Every one of the twelve seasons has a hue lean, warm for the Spring and Autumn families, cool for the Summer and Winter families, and the four true seasons, True Spring, True Summer, True Autumn, True Winter, are defined by hue being their single most extreme, dominant characteristic. Hue is judged independently of how light or dark a color is, value, and how saturated or muted it is, chroma, which is why two colors can share a hue and still look completely different once value or chroma changes.
Why it matters
Hue is usually the first thing people notice about a color and the first question a seasonal analysis answers, warm or cool. Getting hue right immediately narrows the palette down to a workable warm or cool half of the wheel.