Ambisonics records sound as a set of spherical harmonics — essentially, it captures the direction and intensity of sound arriving from all angles. First-order ambisonics uses 4 channels (W, X, Y, Z); higher-order ambisonics uses more channels for greater spatial resolution.
Unlike channel-based surround (5.1, 7.1), ambisonics is speaker-layout independent. The same ambisonics recording can be decoded for stereo, 5.1, 7.1.4, or binaural headphones. Used in VR, 360° video, and immersive audio production.