Visible Light
Visible light is the narrow band of the electromagnetic spectrum the human eye can detect, from red (about 700 nm) to violet (about 400 nm).
Definition
Visible light is the light the eye can see, made up of all the colors of the rainbow mixed together, red, orange, yellow, green, blue, and violet, and objects do not have color on their own: a red apple looks red because it bounces red light back to the eye while soaking up the other colors. It is the portion of the electromagnetic spectrum detectable by the human eye, with wavelengths from about $400\ \text{nm}$ (violet) to $700\ \text{nm}$ (red), white light being a mixture of all of them, traveling at $3.00 \times 10^8\ \text{m/s}$ in a vacuum like every electromagnetic wave. Visible light spans photon energies of about $1.8$ to $3.1\ \text{eV}$, the band where the Sun's $5{,}800\ \text{K}$ blackbody spectrum peaks and where Earth's atmosphere is transparent, no coincidence given how eyes evolved; in a medium of refractive index $n$, light slows to $c/n$ and its wavelength shortens to $\lambda/n$ while its frequency stays unchanged, and because $n$ varies with wavelength, a prism separates colors, with color perception arising from three cone types whose overlapping sensitivities make most perceived colors mixtures rather than single wavelengths.
Example
Sunlight looks white, but a prism or a rain shower spreads it into a rainbow, showing all the hidden colors; red light at $700\ \text{nm}$ has frequency $f = c/\lambda = 3.00 \times 10^8 / 7.0 \times 10^{-7} \approx 4.3 \times 10^{14}\ \text{Hz}$, while violet at $400\ \text{nm}$ is $7.5 \times 10^{14}\ \text{Hz}$, almost double, so the visible band is nearly one octave wide. Wien's law puts the Sun's peak at $\lambda_{\max} = 2.90 \times 10^{-3}/5800 \approx 500\ \text{nm}$, squarely in the green, and sunlight entering glass ($n = 1.52$) slows to $1.97 \times 10^8\ \text{m/s}$, shrinking $500\ \text{nm}$ light to $329\ \text{nm}$ inside the glass while it still emerges perceived as green when it exits, since frequency never changed.
Key Insight
Bees see ultraviolet and some snakes sense infrared, so visible describes human eyes, not light itself, since the spectrum does not care where humans happen to stop seeing. The Sun does not look green even though its spectrum peaks there, because it radiates strongly across the whole visible band and the eye integrates the mixture as white; a star that actually looked green would need a spectrum no blackbody can produce.