Kelvin (K)
Light
The unit for the colour temperature of light. The lower the number, the warmer (more yellow/orange) the light.
Practical Kelvin values: ~1800-2500K = candlelight / fireplace. ~2700-3200K = incandescent bulb, warm-white LED. ~3200-4000K = halogen lamp, photo studio. ~5500-6500K = daylight, camera flash. ~7000-8000K = shade, overcast sky, blue sky. Counterintuitively, a "warm" cosy lamp has a low Kelvin value. Your camera lets you set a fixed Kelvin under white balance settings.
Tools that help with this: