Electromagnetic Spectrum
Definition
The full family of light — from long radio waves through visible light to short gamma rays — all the same phenomenon at different wavelengths.
Technical definition
The continuum of electromagnetic waves ordered by wavelength/frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray. All travel at c in vacuum; photon energy E = hf increases with frequency.
Example
Your eyes see only 380–750 nm of it — a tiny slice. Wi-Fi (~12 cm), thermal cameras (infrared), and X-rays are all the same kind of wave, just different wavelengths.
Common confusion
Different names do not mean different physics — radio waves and gamma rays are both EM waves obeying Maxwell's equations. What changes with wavelength is how they interact with matter (and photon energy).